Every computer user needs a good PDF reader, but many users make the mistake of thinking Adobe Reader is the best and/or only option. It's not. In fact, I'd say it's overkill for most folks, offering features you don't need, plodding performance, and enough resource-hogging bloat that it can actually make your system run slower.
Not to worry: just uninstall Reader and replace it with a smaller, faster, less-intrusive PDF viewer. There are plenty of programs that fit this description, but if you're looking for the smallest, fastest, and least intrusive, check out Sumatra PDF.
I'd call Sumatra a bare-bones, quick-and-dirty PDF viewer. It loads in an instant, consumes only a few megabytes of space on your hard drive, and uses just 6MB of system RAM when running (not including whatever documents you load into it). Adobe Reader uses closer to 30MB of system RAM (when you factor in its startup utility, which is always running).
By design, Sumatra has very few features. It lets you view pages in whatever configuration you want (single-page, facing pages, fullscreen, etc.), rotate pages, e-mail pages, and so on. If a PDF has a hyperlink, you can click it to open your browser. Interestingly, it also includes an Open in Adobe Reader option, just in case you encounter a PDF that does require a Reader-specific feature (and assuming you haven't uninstalled Reader).
If you're a fan of running programs from a flash drive, make sure to grab the portable version of Sumatra PDF; it can run directly from an executable, meaning there's nothing to install.
I'm not saying Sumatra PDF is the best Adobe Reader alternative out there, simply that it's one of the smallest and fastest. Plus, it's free (correction: open-source), which is always good. If there's another PDF viewer you like better, name it in the comments!