Sumatra PDF Viewer for Windows
Sumatra PDF is a slim, free, open-source PDF viewer for Windows. Sumatra has a minimalistic design and simplicity has a higher priority than a lot of features. It's also portable i.e. it's just one file with no external dependencies so you can run it from external USB drive with no problems.

Sumatra can do the following things:

• open PDF files via menu
  
• open PDF files via drag & drop
  
• page up and 'p' for going to previous page
  
• page down and 'n' for going to next page
  
• 'q' to quit
  
• set different zoom levels via menu

More Info & Download:
http://blog.kowalczyk.info/software/sumatrapdf/

Sumatra PDF 0.9.4

Changes

- Improved PDF compatibility (more types of documents can be rendered)

- Added settings dialog (contributed by Simon Bünzli)

- Improvements in handling unicode

- Changed default view from single page to continuous

- SyncTex improvements (contributed by William Blum)

- Add option to not remember opened files

http://blog.kowalczyk.info/software/sumatrapdf/download.html