You don't have to use Adobe Reader to open pdf files (not that there's anything wrong with that). Lifehacker has a blogpost that compares the features of what it calls the "five best pdf readers". Why would you want to use a different pdf reader? There are readers you might want to consider that are faster, lighter, opensource, and that have advanced tools for annotation etc. For example, I've been using PDF-Xchange Viewer (one of the readers that made the list) for a few years; I like it because it takes up less space than Reader (17 mb vs. 134 mb) and has annotation tools that let me add text, sticky notes, etc. to pdfs.
The basic readers in the article are free; most have professional upgrades with more features that you can purchase.