pdf.js

Pdf.js is the library that Firefox itself uses to render PDFs. It parses and renders PDF documents entirely in JavaScript, no plugins or native code required.

Fully free under Apache 2.0. Mozilla maintains it actively. You get a complete PDF viewer with zoom, search, text selection, page navigation, and form filling. Drop it into any web app and you have a functional PDF reader. It powers Firefox's built-in PDF viewer, so it handles edge cases most alternatives can't.

The catch: performance on large PDFs (100+ pages, heavy graphics) can be slow compared to native renderers. Rendering fidelity isn't perfect. Some complex PDFs with unusual fonts or annotations won't look identical to Acrobat. And if you need PDF generation (creating PDFs, not displaying them), this isn't it. Look at react-pdf or jsPDF for that. pdf.js is display-only, and it's the best at that specific job.