pdf.js

pdf.js is the PDF renderer that Firefox uses, and it's the reason you can view PDFs in your browser without Adobe Reader. Pure JavaScript, no plugins, renders directly to Canvas or SVG. It's the default PDF viewer for the web.

If you're building a web app that needs to display PDFs — document management, contract signing, report viewing — pdf.js is the starting point. PSPDFKit and PDF Tron are the commercial options with annotation, form filling, and signature features built in. React-PDF wraps pdf.js for React components. Commercially, Adobe's PDF Embed API is free but adds a dependency.

53K+ stars, maintained by Mozilla, and running in production in literally billions of browser sessions. The rendering quality handles most PDFs correctly.

The catch: pdf.js renders PDFs — it doesn't edit them. No built-in annotation, form filling, or digital signatures. Complex PDFs with advanced features sometimes render imperfectly. Performance on very large documents (500+ pages) can lag. And if you need server-side PDF processing (generation, manipulation), you want a different tool entirely — look at pdf-lib or Puppeteer.