If everybody recognizes how useful PDF files are, most also agrees that Adobe Acrobat reader is big (download package is 22 MB) and really slow to load.

For a fast opening of PDF files, Foxit, a free, light and fast viewer often replaces Acrobat Reader but Foxit still requires an installation package (2 MB).

Sumatra PDF (beta 0.7), a new comer, is a free, light (0.8 MB), opensource, and single executable file which can easily be installed either on a computer or on an USB stick. 

 Via : Lifehacker