SumatraPDF

The lightest PDF reader for Windows. Opens PDFs, ePubs, MOBIs, CBZs, and more — instantly.

Windows Open Source Writing

SumatraPDF is a tiny, portable PDF reader that opens documents faster than anything else on Windows. Supports PDF, ePub, MOBI, CHM, XPS, DjVu, and comic book formats (CBZ/CBR). Single executable, no installation needed, starts in under a second. Free, open-source, and minimal. No toolbars, no ads, no bloat. Just a clean window with your document. The default choice for anyone who values speed over features.