sumatrapdf

SumatraPDF is a free, lightweight document reader for Windows. It can open a wide range of file formats including PDF, EPUB, MOBI, CBZ, CBR, FB2, CHM, XPS, and DjVu, making it a one-stop tool for reading almost any document or e-book format on a Windows computer. It is released under the AGPL v3 license, meaning it is open-source and free to use. With nearly 17,000 GitHub stars, it is a well-regarded alternative to heavier PDF readers. The source code is written in C and C++. More details, a manual, and contributor information are available on the official SumatraPDF website.