Sumatra PDF is a slim, free, open-source PDF viewer for Windows.
Why another PDF viewer?
There's nothing wrong with Adobe Acrobat... except for annoyingly long startup time. 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.
Version history 0.4 [2007-02-19]
* ask before registering as a default handler for PDF files
* faster rendering thanks to alternative PDF rendering engine. Previous engine is available as well.
* scrolling with mouse wheel
* fix toolbar issues on win2k
* improve the way fonts directory is found
* improvements to portable mode
* uninstaller completely removes the program
* changed name of preferences files from prefs.txt to sumatrapdfprefs.txt