At some point every developer ends up needing to lock down a PDF — an invoice with client banking details, an internal report, a contract that shouldn't sit unprotected in a shared drive. You don't need Acrobat Pro's $22.99/month subscription for this. Password protection with AES encryption is either free, built into your OS, or a five-minute open-source install.
Here are 10 ways to do it, from browser-based tools to the CLI.
1. ToolTiny — straightforward AES-128 encryption, no account
ToolTiny's Protect PDF tool does exactly one thing well: upload a PDF, type a password twice, download the encrypted file. It uses 128-bit AES encryption, doesn't store your password or file (there's genuinely no recovery option if you forget it — that's a security feature, not a bug), and auto-deletes everything from its servers shortly after processing. File size cap is 20MB, which covers the vast majority of contracts and reports but rules out large scanned documents.
2. iLovePDF — batch protect multiple files at once
If you need to lock down 15 invoices instead of 1, iLovePDF lets you upload and encrypt multiple PDFs in a single pass, which most single-file tools (including ToolTiny) don't support. No account needed for basic use.
3. Smallpdf — clean UI, files auto-deleted after 1 hour
Smallpdf's encryption tool uses AES-128 over a TLS connection and explicitly deletes files an hour after processing. The interface is arguably the most polished of the online options here, though the free tier's task limits mean it's better suited to occasional use than a recurring workflow.
4. PDF24 — no limits, no watermarks, works online or as desktop software
PDF24 stands out for having no aggressive free-tier restrictions — you can protect as many PDFs as you want without hitting a wall. It's also available as installable desktop software if you'd rather not upload files to a server at all.
5. Adobe Acrobat Online — free with account, most granular control
Adobe's password protection is free once you create an account (no credit card), and it gives you more control over permissions than most competitors — separately restricting printing, copying, or editing in addition to the open password. If you need more than that, a 7-day free trial of Acrobat Pro unlocks certificate-based encryption and permission management.
6. LightPDF — batch protection plus AI extras
Similar batch-processing capability to iLovePDF, with the addition of AI-powered features like PDF summarization and translation bundled into the same platform if you're already looking for a broader PDF toolkit.
7. Wondershare PDFelement Online — enterprise-grade, G2-rated
PDFelement's online protector supports both AES-128 and AES-256, and the desktop version adds permission restrictions (no editing, printing, or copying) on top of the password itself. It's a heavier product aimed more at business users than a quick one-off task.
8. macOS Preview — built-in, zero install, AES-128
If you're on a Mac, you already have a PDF encryptor installed. Open the file in Preview, go to File → Export, check Encrypt, and set a password. It's AES-128 and completely offline — nothing touches a server, ever.
9. Microsoft Word — encrypt on export, AES-128
Less known: if you're exporting a Word document to PDF, Word's File → Info → Protect Document → Encrypt with Password applies before the PDF is even created, so the encryption travels with the export. Works the same way in LibreOffice, which as of recent versions (25.8+) supports AES-256 — actually stronger than what Word or most free online tools default to.
10. qpdf — the open-source, scriptable option
For anyone who wants to protect PDFs as part of an automated pipeline rather than clicking through a UI, qpdf is a free command-line tool that handles PDF encryption directly:
qpdf --encrypt <user-password> <owner-password> 256 -- input.pdf output.pdf
That one line applies AES-256 encryption with separate user and owner passwords — useful if you want "can open" and "can edit/print" to require different credentials. It's the only option on this list that fits naturally into a CI job or a batch script.
Quick comparison
| Tool | Encryption | Batch support | Account needed | Best for |
|---|---|---|---|---|
| ToolTiny | AES-128 | No | No | Quick one-off files |
| iLovePDF | AES-128 | Yes | No | Multiple files at once |
| Smallpdf | AES-128 | Limited | No | Occasional, polished UX |
| PDF24 | AES-128 | Yes | No | Repeated use, no caps |
| Adobe Acrobat | AES-128/256 | Yes | Yes (free) | Granular permissions |
| LightPDF | AES-128 | Yes | No | PDF + AI tools bundled |
| PDFelement | AES-128/256 | Yes | Depends | Business/enterprise use |
| macOS Preview | AES-128 | No | No | Offline, already installed |
| Word/LibreOffice | AES-128/256 | No | No | Encrypting during export |
| qpdf | AES-256 | Yes (scriptable) | No | Automation/CI pipelines |
The honest caveat
A password on a PDF is access control, not tracking. None of the tools above — including the paid ones — let you revoke access after you've sent the file, see who opened it, or stop someone from forwarding it once they have the password. If that level of control actually matters (distributing a confidential data room, tracking who's viewed a legal doc), you're looking at a different category of tool entirely — identity-bound access platforms, not password encryption. For the much more common case of "keep this out of the wrong hands in transit," a plain AES-encrypted PDF from any tool on this list is genuinely sufficient.
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