PDF.js. A general-purpose, web standards-based platform for parsing and rendering PDFs. Developed by Mozilla Labs
License: Open Source
MuPDF is a free and open-source software framework written in C that implements a PDF, XPS, and EPUB parsing and rendering engine. It is used primarily to render pages into bitmaps, but also provides support for other operations such as searching and listing the table of contents and hyperlinks. Developed by Artifex Software, Inc.
License: Open Source
Firefox PDF Viewer (PDF.js) is designed for integration into web browsers and emphasizes accessibility and ease of use, while MuPDF is a more performance-oriented solution with a focus on rendering quality. Both are open-source and free, but they cater to different needs and use cases.