Foxit Reader is a free PDF document viewer, with incredible small size, breezing-fast launch speed and rich feature set. Its core function is compatible with PDF Standard 1.7. In the past, you've had to download a huge PDF reader from another software company, go through a lengthy installation process and wait for an annoying splash window to disappear just to open a PDF document. 
Adobe Reader is a real pain in the hindquarters. It's monstrously large, slow to load, and includes many features most users will hardly ever need. Foxit PDF Reader 3.0 kills the bloat and throws in some useful features, such as multimedia support and content-sharing options, while still allowing you to quickly access your PDFs.
The interface mimics Adobe's, so you won't have to change your reading habits. The small program starts surprisingly fast compared with Adobe. You might need to fiddle a bit to get it to work with your Internet browser, but you can find help on Foxit's support forums. It's a nice touch that it opens PDFs from the Internet in their own Foxit window, instead of sucking resources from within the browser.
