Merge PDFs in the browser with JavaScript (no uploads, no server)

In this post I'll show how to merge PDF files entirely in the browser using PDF.js and pdf-lib — no server, no file upload, no backend. Everything runs on the user's machine, which is great for privacy and for keeping hosting costs at zero (it's just a static site). Why process PDFs on the client? Most "free" PDF websites quietly upload your documents to their server, which: Exposes private/sensitive files to third parties Imposes size limits Often slaps a watermark on the output Requires you to trust their storage If you handle PDFs with client-side JavaScript (WebAssembly / WASM + PDF.js), none of that happens. The user's file never leaves their device, and you don't need a backend at all — so it's cheap and private. Caveats pdf-lib works well with standard PDFs; heavily encrypted or unusual documents may need extra handling. Very large PDFs are memory-hungry since everything is client-side, but for typical documents it's fast and free. Some complex PDFs with unusual fonts can lose fidelity — test on your own files first. Try it I packaged this approach (plus split, compress, rotate, unlock, image-to-PDF) into a free no-upload tool: https://yourutilityhub.com/pdf/merge-pdf The whole project is open source: https://github.com/Jalal-khn/utilityhub- If you have questions about the architecture or want a deeper dive on any part, ask away. The basic idea Read the input file with FileReader Parse it with pdf-lib (a pure-JS PDF library) Copy the source pages into a new document Save the merged PDF and trigger a download Here's the core function: js import { PDFDocument } from "pdf-lib"; async function mergePdfs(files) { const merged = await PDFDocument.create(); for (const file of files) { const bytes = await file.arrayBuffer(); const src = await PDFDocument.load(bytes, { ignoreEncryption: true }); const pages = await merged.copyPages(src, src.getPageIndices()); pages.forEach((page) => merged.addPage(page)); } const out = await merged.save(); return new Blob([out], { typ