From Krzysztof Kowalczyk
Sumatra PDF is a slim, free, open-source PDF reader for Windows. Portable out of the box.
Why another PDF reader?
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: only one file so you can run it from external USB drive. Doesn't write to registry.