Note: This review addresses v0.8.1 of the software.
Adobe Reader is a bloated, buggy piece of software that's far more trouble than it's worth. With the free Sumatra PDF, you can throw it away forever. Unlike Adobe Reader, it's slim and takes up very little RAM and few system resources. It doesn't "phone home" in search of updates. And it most likely won't crash your system. What it will do is load PDF files very, very quickly. And it won't cost you a penny.
Sumatra PDF doesn't have as many features as another popular alternative PDF program, Foxit Reader, offers for tasks such as for file navigation. But the Sumatra download is half the size of Foxit (which is pretty small already). And it's far superior to the Adobe product. Sumatra PDF does a very nice, straight-ahead job of PDF viewing.