Sumatra PDF (http://blog.kowalczyk.info/software/sumatrapdf/free-pdf-reader.html) is a slim, free, open-source PDF viewer for Windows. Portable out of the box.

Ver.1.1 -> Ver.1.2 (2010-11-26)
- improved printing: faster and uses less resources
- add Ctrl-Y as a shortcut for Custom Zoom
- add Ctrl-A as a shortcut for Select All Text
- improved full screen mode
- open embedded PDF documents
- allow saving PDF document attachements to disk
- latest fixes and improvements to PDF rendering from mupdf project

Screenshot:
http://img230.imageshack.us/img230/3112/sumatrapdf.th.png (http://img230.imageshack.us/i/sumatrapdf.png/)