Sumatra PDF

Sumatra PDF is a slim, free, 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.

Homepage: http://blog.kowalczyk.info/software/sumatrapdf/