Sumatra PDF Is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR Reader for Windows

Sumatra is my favorite PDF viewer on Windows. The fact that it can display all these other formats as well is a nice bonus.

It's much more lightweight and unobtrusive than Adobe's Reader or Foxit, both in terms of resources and UI. Also, it can run without installing it, so one does not need any special privileges to use it.

Last but not least, it remembers the position in a document across opening/closing it.