DEV Community: OpenPdfEdit

Two ways to save a PDF, and why redaction uses the other one

OpenPdfEdit — Mon, 26 Oct 2026 12:30:46 +0000

A PDF is not one long run of text. It is a set of numbered objects — pages, fonts, the content streams that paint each page, annotations — with a table at the end saying where each one starts. That structure allows two very different ways of saving a change, and OpenPdfEdit uses both on purpose.

Appending: what most edits do

The format lets a file change without touching what is already in it. The new and changed objects are written after the end of the file, followed by a new table that points at them. This is called an incremental update. A reader follows the newest table, so it shows the edited document. The older objects are still there, earlier in the file.

OpenPdfEdit saves this way for almost every edit. A highlight, a sticky note, a replaced line of text, a rotated or deleted page: each one compares the document with how it stood at the last save and appends only the objects that changed. The bytes before that point are never rewritten.

That has two practical results. A digital signature already in the file survives an annotation, because the bytes it covers are never touched. And each save is a revision on top of the last one, not a new file that quietly replaces it.

Why that is wrong for redaction

The property that protects a signature defeats a redaction. Redacting in OpenPdfEdit removes the text from the page. The page's content stream is interpreted, and whatever paints inside the box you drew is taken out. Text is measured with the font's real widths where they are known; where they are not, every piece of text the box touches goes whole. Either way the rule errs toward removing too much, never too little, so draw the box over exactly what you mean to remove, and open the saved copy to check it before you send it. The black box drawn on top is a second line of defence, not the mechanism.

Now picture that change appended. The page would point at a new content stream, while the old one — the one with the text in it — stayed in the file, one revision back. Anything that reads the raw file could still find it. The removal would be real, and the file would still carry a copy of what was removed.

So redaction rewrites the whole file

When you redact, OpenPdfEdit writes the document from scratch instead. Before it does, it drops every object that nothing in the document points to any more. That step is the one that matters. Redaction leaves the old content stream orphaned, and a plain rewrite would still write the orphan out. Pruning it is what makes the removal stick.

Find & redact follows the same rule. Every match you keep after reviewing the list is removed in one step: one rewrite of the file, one entry in the undo history. Then the app reads the saved file back. If any redacted area still has readable text in it, it names the page and asks you to deal with it, rather than counting the job as done.

What it costs

A full rewrite breaks an existing digital signature. That is not a side effect of rewriting. A signature exists to show that the signed bytes have not changed, and redaction changes them. Redacting a signed document was always going to invalidate the signature, whichever way the file was written.

Compress is the other operation that writes a file from scratch. It saves a new copy and leaves the open document alone. Even at its lossless level, that copy drops the chain of revisions appended edits build up, along with any orphaned objects. Signatures do not carry over to that copy either.

Checking it yourself

All of this happens on your own machine, in the desktop app or in the browser. The document is never uploaded. To check a redaction, open the saved file in any PDF reader, search for the words you removed, and try copying the text around where they were. Neither should find them.

The code that chooses between the two save paths is public, in the openpdfedit-doc and openpdfedit-session crates of the source on GitLab, under AGPL-3.0-or-later. How to redact a PDF walks through the tool itself.

Originally published at openpdfedit.com.

Electronic signature vs digital signature

OpenPdfEdit — Mon, 26 Oct 2026 12:22:29 +0000

Electronic signature vs digital signature is a confusing pair, because the two names sound like one thing. They are two different things. An electronic signature is a mark that shows you agree to a document. A digital signature is a piece of cryptography attached to the file. A PDF can carry either, or both.

Electronic signature vs digital signature, in short

An electronic signature is about intent. It is your name on the page, put there to say yes.
A digital signature is about the file. It is made with a certificate, and it lets a reader check whether the document has changed since it was signed.

A digital signature is one way of signing electronically, so every digital signature is also an electronic one. It does not work the other way round: a drawn signature is not a digital signature.

What an electronic signature is

An electronic signature is any mark made on a screen that stands for your agreement. In a PDF that usually means one of three things: a signature drawn with a mouse or a finger, a name typed in a handwriting font, or a picture of the signature you write on paper.

All three end up as something you can see on the page. None of them involves a certificate, and nothing in the file checks who put the mark there or whether the text around it changed later. It is the on-screen version of signing a printout.

That is enough for a great many everyday documents, and it is quick: you can e-sign a form in a minute without printing anything.

What a digital signature is

A digital signature is made with a private key that belongs to a certificate. The software calculates a fingerprint of the file's bytes and seals it with that key. The seal and the certificate are stored inside the PDF.

Afterwards, a reader that supports digital signatures can do two things. It can recalculate the fingerprint and see whether the file still matches, so a change made after signing shows up. And it can look at the certificate to see who it was issued to, and whether it trusts whoever issued it.

A digital signature PDF often shows a visible signature box as well, but the box is decoration. The part that matters is the seal you cannot see.

Electronic signature vs digital signature: which one do you need?

It depends on who is asking, and this page cannot tell you what a particular contract, office or country requires. The requirements differ from one situation to the next, and they are set by whoever receives the document or by the rules that apply where you are. If you are not sure, ask the person who sent it.

As a rough guide to what people mean:

If you were told to “sign and return” a form, a visible signature on the page is usually what is expected.
If you were told the document must be “digitally signed”, “signed with a certificate” or signed through a named signing service, a drawn signature will not meet that, however good it looks.

Which one OpenPdfEdit makes

In terms of electronic signature vs digital signature, OpenPdfEdit, a free open source PDF editor, makes the first kind. You draw a signature, type your name or load a picture, and place it on the page as a visible mark. The sign a PDF page describes the tool, and how to add a signature to a PDF has the steps.

It does not create digital signatures. It has no certificates and does no cryptographic signing.

It does not verify them either. When a document already has digital signatures, the app lists what the document itself declares about each one: the signature format, and the name, reason and signing time the signer supplied. It also checks that the signed byte range has the shape a well-formed signature must have. That is a list of claims. It does not prove that a signature is valid, trusted, or tied to the bytes it says it covers, and the app labels it as not cryptographically verified. To verify a digital signature, use software that checks certificates.

Adding markup to a document that is already digitally signed

A digital signature covers the bytes of the file as they were when it was made. So what matters is how an edit is saved.

OpenPdfEdit saves a highlight, a note or a placed signature by appending the change to the end of the file. The bytes the existing signature covers are not touched. Some edits cannot be saved that way. Redaction rewrites the whole file, on purpose, and a rewrite breaks an existing digital signature. Both ways of saving are explained in two ways to save a PDF.

FAQ

Is an electronic signature PDF the same as a digitally signed PDF?

No. An electronic signature PDF has a visible signature on the page. A digitally signed PDF has a certificate-based seal inside the file, which a reader can check. That is the whole of electronic signature vs digital signature: a mark on the page, or a seal on the file.

Can I tell which one a PDF has?

Open it in a reader that supports digital signatures. A digitally signed document shows a signature panel or a banner about its signatures. A drawn signature is simply part of what you see on the page.

Does OpenPdfEdit make digital signatures?

No. It places a visible electronic signature on the page. It does not create or verify certificate-based digital signatures.

Which one should I use?

Whichever the recipient asks for. When you weigh electronic signature vs digital signature for a real document, the deciding question is what the other party, or the rules that apply to you, require.

Open the app

Originally published at openpdfedit.com.

How to add a signature to a PDF

OpenPdfEdit — Mon, 26 Oct 2026 12:15:15 +0000

This guide shows how to add a signature to a PDF without printing it, signing the paper and scanning it back in. You make the signature once, by drawing it, typing your name or using a photo of the one you write by hand. Then you put it on the page and save.

The steps use OpenPdfEdit. It is free, it needs no account, and it does the work on your own computer, so the document is not uploaded anywhere. There is a summary of what the tool can do on the sign a PDF page.

How to add a signature to a PDF: the short answer

Open the PDF, switch to Annotate and choose Signature. Choose New signature and make one with Draw, Type or Image. Tap the page where the signature belongs, then save.

Before you start

Open openpdfedit.com/app and choose Open PDF…. The desktop app and the browser extension work the same way.
Switch to Annotate at the top of the window.
Choose Signature. The first time, the Signatures panel opens, because there is no saved signature yet.
Choose New signature. The dialog that opens has three tabs: Draw, Type and Image.

Use whichever of the three suits you. You only do this part once.

Option 1: draw your signature

Stay on the Draw tab.
Write your signature in the box with a mouse, a trackpad, a finger or a pen. The faint line is only a guide and is not saved.
If it did not come out well, choose Clear and write it again.
Give it a name if you like, then choose Save signature.

The drawing is cropped to the ink, so it does not matter where in the box you wrote or how small.

Option 2: type your name

Open the Type tab.
Type your name.
Pick one of the three handwriting styles: Dancing Script, Great Vibes or Caveat. Each one shows your name as it will look.
Choose Save signature.

The name is saved as a shape and not as text in a font, so it looks the same in every PDF reader.

Option 3: use a picture of your signature

Sign a sheet of white paper and take a photo of it, or scan it. Save it as a PNG or a JPEG.
Open the Image tab and choose Choose an image….
Leave Remove the white background ticked, and move the Threshold slider until the paper has gone from the preview and the ink is still there.
Choose Save signature.

The picture is cropped to the ink, and the removed background stays transparent. On the page the signature sits on its line without a pale rectangle around it.

Place the signature on the page

Pick your signature in the Signatures panel. The panel closes and the signature is ready to place.

Tap the page where the signature should go. It is placed at a standard size, centred on the spot you tapped.
Or drag a box to choose the size yourself. The signature is fitted inside the box at its own proportions, so it is never stretched.

A drawn or typed signature is placed in the ink colour chosen in the panel. A picture keeps its own colours. To use a different saved signature, choose Signature again.

If it is in the wrong place or the wrong size

The position and the size are both set at the moment you place the signature: where you tap, or how far you drag. If it landed in the wrong spot, undo and place it again. The signature is already saved, so a second try takes a moment.

Save the document

Save the PDF when the signature is where you want it. In Chrome and Edge the web app can save over the file you opened. Other browsers save a copy. The desktop app saves over the original.

The signature itself stays in the Signatures panel on your device. The next time you need to add signature to PDF pages, skip straight to placing it.

Should you flatten it afterwards?

The signature is stored as a separate object on top of the page. Anyone who opens the file in a PDF editor can select that object and delete it. If the signature has to stay where it is, flatten the document, which makes it part of the page. The steps are in how to flatten a PDF. Keep an unflattened copy if you might need to change something later.

What this kind of signature is, and is not

What you have added is a visible mark, the same as signing a printout. It is not a digital signature made with a certificate: nothing in the file ties it to your identity or shows whether the document changed afterwards. OpenPdfEdit does not make that kind of signature. If you are not sure which one you have been asked for, read electronic signature vs digital signature.

FAQ

Can I add a signature without printing the document?

Yes. That is the point of this guide on how to add a signature to a PDF: the signature is drawn, typed or loaded from a picture, and placed on the page on screen.

Can I sign a PDF for free?

Yes. Making and placing signatures is free and works without signing in. Nothing is stamped on the page besides your signature.

How do I sign a PDF without Adobe Acrobat?

Any PDF editor that can place a signature will do. The steps above use OpenPdfEdit in a browser tab, with nothing to install.

How do I insert a signature in a PDF form?

Fill in the form first, then place the signature on the signature line in the same way. There is more about forms on the page about how to fill PDF forms.

Is my signature uploaded?

No. A saved signature is kept on your own device, and the document is signed on your own computer.

Open the app and sign your PDF

Originally published at openpdfedit.com.

How to compare two PDFs and see what changed

OpenPdfEdit — Mon, 26 Oct 2026 12:06:35 +0000

To compare two PDF files, open the first one in OpenPdfEdit, choose Compare, and pick the second. A panel lists each line of text that differs, page by page, with the old wording crossed out and the new wording beside it. Click an entry and the app takes you to that place in the document.

It runs in your browser tab. Neither file is uploaded, and you do not need an account.

Step by step

Open openpdfedit.com/app and choose Open PDF…. Pick the older of the two versions.
In the toolbar, choose Compare. It is in the Document group while the app is in Read. In Annotate or Edit, look under All tools.
A file dialog opens. Pick the newer version.
The Compare panel opens on the right. Its first line names the file you compared with. The number beside the title is how many differences were found, and they are listed under a heading for each page.

If nothing differs, the panel says "No differences found."

You can also start from the first screen, before any file is open. Choose Compare in the list of tools, pick the first file, then the second.

How to read the list

Each entry is one of four kinds.

A changed line. The line from the open document is shown in red and struck through. After the arrow comes the same line from the other file, in green and underlined:

The Client will pay the Supplier a fixed fee of USD 12,000 for the services, plus any expenses → The Client will pay the Supplier a fixed fee of USD 15,000 for the services, plus any expenses

The whole line is shown, and the changed word is not marked on its own, so read the two versions against each other. Small edits are listed as well: one digit, a missing comma, a capital letter.

Red only. The text is in the open document and not in the other file. If you opened the older version first, this text was removed.

Green only. The text is in the other file and not in the open one. It was added.

"Drawn differently; the text is the same". The words on that page match, but the page does not look the same. You get this when a picture was replaced or moved, or when a signature or a highlight was added.

Click an entry to go to it. For a changed line or for red text, the app outlines the lines on the page. For a page that is drawn differently, it shades the area that differs.

Green-only text is not in the open document, so there is nothing to outline. The app shades the place where that text sits in the other file, and in the open document other text may be there. To see added text where it belongs, run the comparison the other way round: open the newer version and compare it with the older one. The same passage is then listed in red and outlined on the page.

If the files do not have the same number of pages, the panel says so at the top, for example "This document has 3 pages; the other has 4." A page that exists only in the second file is marked "Only in the other document".

Keeping a record of the changes

The list exists only on screen. The panel has no export, and closing it discards the list. To see it again, choose Compare and pick the file again.

Comparing does not change either file.

If you need something to send or to file, mark the changes on the document yourself:

With the Compare panel open, switch to Annotate. The panel stays where it is.
Click an entry to jump to it, choose Highlight in the tool list on the left, and drag across the line.
Repeat for the other entries, then choose Save a copy. The copy is offered under the same file name as the original, so change the name to keep both.

There is more about the markup tools in annotating a PDF.

What the comparison cannot tell you

It compares page 1 with page 1, page 2 with page 2, and so on. When new text pushes a paragraph onto the next page, that paragraph is listed in red on one page and in green on the next, although its wording has not changed. When a whole page was inserted, every page after it is listed as changed. So if a red entry on one page is matched by the same text in green on the following page, the text has moved. It has not been rewritten.

It compares line by line. If the second file was laid out differently, with another font size or other margins, the lines break in different places. They are then listed as changed even where the words are identical. The result is cleanest when both files were made the same way.

Scanned pages have no text to compare. A scan is a picture of a page. Compare a scan with an ordinary PDF and the full text of every page is listed as a difference. Compare two scans and the list can only say which pages look different. Two scans of the same sheet do not match dot for dot, so expect every page to be listed.

Your files stay on your computer

OpenPdfEdit compares the files inside the browser tab. Both PDFs are read from your disk. Nothing is uploaded.

It also works without a connection. Open the app once while you are online and let it finish loading. After that you can reload the page with the network off and compare as usual.

FAQ

How do I compare PDF files for free?

Open the first file in OpenPdfEdit, choose Compare, and pick the second. Comparing is free, and there is no account to create.

Are my PDFs uploaded when I compare them?

No. Both files are read and compared inside the browser tab.

Does it show the two PDFs side by side?

No. One document is open, and the differences are listed in a panel beside it. Clicking a difference takes you to it on the page.

Can I save or export the comparison?

Not as a report. Highlight the changed lines in Annotate and save a copy, as described above.

Can I compare scanned PDFs?

Not by their text, because a scan has none. The list can only show which pages look different.

Does comparing change my files?

No. Both files are left as they were.

Can I compare PDFs offline?

Yes. Open OpenPdfEdit once while you are online. After that it works with the network switched off.

Open the app and compare your PDFs

Originally published at openpdfedit.com.

How to merge PDF files offline, without uploading them

OpenPdfEdit — Mon, 26 Oct 2026 12:05:03 +0000

Most "merge PDF" websites work by upload. You send your files to a server, it joins them, and you download the result. That is fine for a restaurant menu. It is less fine for a contract, a payslip or a scan of your passport, and it is no use at all on a plane.

Merging does not need a server. A PDF is a set of pages, and joining two of them is something a browser tab can do by itself, on your own computer.

The short answer

Open OpenPdfEdit in your browser once while you are online. After that the tab works without a connection. Choose Merge PDFs…, pick your files, put the pages in order and save a copy. The files are read from your disk and the merged file is written back to it. Nothing is uploaded, and you do not need an account.

Step by step

Open openpdfedit.com/app. Do this once with a connection, so your browser can keep its own copy of the app.
On the start screen, choose Merge PDFs… and pick the PDF that should come first.
The app asks whether to include the document you have open in the merge. Choose Include it.
Pick the other PDFs. You can select several at once. Their pages are added after the first file.
A new document called merged.pdf opens with all the pages in it. Open Pages to see them as thumbnails, and drag a thumbnail to move that page.
Choose Save a copy to save the merged file.

The merge makes a new document. The files you started with are not changed.

Using it with no connection

The first visit is the only one that needs the internet. To see this for yourself, load the app, turn off Wi-Fi and reload the page. The start screen still opens, and the steps above work the same way.

This is useful in two cases. One is the obvious one: a train, a flight, a hotel with bad Wi-Fi. The other is a document you are not supposed to send to an outside service at all, where an upload site is not an option even with a good connection.

How to check that nothing is uploaded

You do not have to take this on trust. Open your browser's developer tools (F12 on most keyboards), switch to the Network tab, and merge two files while you watch.

You will see the app's own files being loaded from openpdfedit.com. You will not see an upload, and you will not see a request to any other site. The PDFs go from your disk into the tab and back to your disk.

Getting the pages in the right order

Merging puts whole files one after another. Often that is not quite what you want: the signed page belongs at the end, or the cover letter should come first.

That is what the Pages panel is for. Every page of the merged document is a thumbnail there, and you can drag any of them to a new place before you save. There is more about rearranging, rotating and removing pages in organising PDF pages.

If the merged file is going to someone else, two other tools are worth knowing about. Password protection puts a password on the copy you send. And if you later need one part of the file back by itself, splitting is the same job in reverse.

If you would rather not use a browser

On a Mac, the Preview app that comes with macOS can also join PDFs: open one file, show the thumbnail sidebar, and drag the second file into it. Windows has no built-in way to merge PDFs, so there you need a tool of some kind.

FAQ

Can I merge PDF files without an internet connection?

Yes. Open OpenPdfEdit once while you are online so the browser can store the app. After that you can merge with the network switched off.

Are my files uploaded when I merge them?

No. The files are read and joined inside the browser tab, and the result is saved straight to your computer. You can confirm it in the Network tab of your browser's developer tools.

Does merging cost anything or need an account?

No. Merging is free and works without signing in.

Does merging change my original PDFs?

No. The merge creates a new document called merged.pdf. Your original files stay as they were.

Can I change the page order before saving?

Yes. Open the Pages panel and drag the thumbnails into the order you want, then save a copy.

Open the app and merge your PDFs

Originally published at openpdfedit.com.