The latest articles on DEV Community by PDF Mergely (@pdf_mergely_7de2f5db895f0). https://dev.to/pdf_mergely_7de2f5db895f0 https://media2.dev.to/dynamic/image/width=90,height=90,fit=cover,gravity=auto,format=auto/https:%2F%2Fdev-to-uploads.s3.us-east-2.amazonaws.com%2Fuploads%2Fuser%2Fprofile_image%2F4009135%2F4b3d72c6-a689-4002-86d0-c18ac79b3fb4.png https://dev.to/pdf_mergely_7de2f5db895f0 en PDF Mergely Tue, 30 Jun 2026 07:12:59 +0000 https://dev.to/pdf_mergely_7de2f5db895f0/how-i-built-pdf-tools-that-run-entirely-in-your-browser-no-uploads-no-servers-3lim https://dev.to/pdf_mergely_7de2f5db895f0/how-i-built-pdf-tools-that-run-entirely-in-your-browser-no-uploads-no-servers-3lim <p>Most online PDF tools work the same way: you choose a file, it uploads to a server, the server does the work, and you download the result. For a flyer that's fine. For a contract, a payslip, or a bank statement, it means handing your document to someone else's computer and trusting what happens next.</p> <p>I wanted to build a PDF toolkit where that question never comes up — so I moved the code to the file instead of the file to the code.</p> <h2> The core idea </h2> <p>A normal web app keeps its logic on a server, so your file has to travel to it. I flipped that around. Modern browsers can read the files you choose and run real processing locally — instead of sending your file to my code, I send my code to your browser. The PDF engine downloads once, then every operation runs on your device.</p> <p>There's no upload step because there's nowhere to upload to. The part of a normal service that would receive your file simply doesn't exist.</p> <h2> How it actually works </h2> <ul> <li>The file stays in memory. The browser hands the app the bytes directly via the File API — they live in the tab, not on a server.</li> <li>Web Workers do the heavy lifting on a background thread, so the UI stays responsive on large documents.</li> <li>WebAssembly runs the fast parts — like AES encryption for password-protecting a PDF — at close to native speed.</li> <li>A strict Content-Security-Policy locks the page down so file data has no network path out. Open the Network tab and watch: nothing leaves while you work.</li> </ul> <p>The easiest proof for a skeptical user: after the first load, it works fully offline. You can merge PDFs on a plane.</p> <h2> The honest tradeoffs </h2> <p>This isn't magic. Because the work happens on your device, very large files depend on your machine's memory, and a phone won't be as quick as a desktop. I design around it — for example, processing one page at a time to keep memory in check.</p> <h2> What's there today </h2> <p>Merge, split, reorder, rotate, delete and extract pages, compress, watermark, page numbers, and password protect/unlock. All free, no sign-up, and it runs on mobile in any modern browser.</p> <p>Try it: <a href="https://pdfmergely.com" rel="noopener noreferrer">https://pdfmergely.com</a></p> <p>I'd love feedback from other builders — especially on the client-side architecture, and on which tool to add next (OCR and edit/sign are the most requested so far).</p> webdev javascript privacy showdev