Sumatra PDF is a slim, free, open-source PDF, DjVu, XPS, CBR, CBZ viewer for Windows. Sumatra has a minimalistic design. Simplicity has a higher priority than a lot of features. It's small and starts up very fast. Options are a little thin on the ground but SumatraPDF provides a very comfortable environment for reading PDFs and you can print documents without much hassle. You can take this over Acrobat any day for looking at eBooks.
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. This classifies it as a portable application. As is characteristic of many portable applications, Sumatra takes up little disk space. It has a 2 MB setup file, compared to Adobe Reader's 45 MB, for Windows XP, Vista and 7.