Sumatra PDF is a open-source PDF viewer for Windows. Sumatra has a minimalistic design. Simplicity has a higher priority than a lot of features. It's small and starts up very fast. It's designed for portable use: it's just one file with no external dependencies so you can easily run it from external USB drive. You can read PDF, eBook (ePub, Mobi), XPS, DjVu, CHM, and comic books.