Acrobat started out with some very ambitious ideas, which is part of the reason Adobe's reader is crufty and bloated.
Originally, the PDF specification called for a lot more than just a file format for reading documents. PDF files can contain all sorts of things--annotations, sound, multimedia, executable programs and applets(!), JavaScript, event triggers, all sorts of stuff.
Now, almost nobody every uses those capabilities--but they're still present in Reader nonetheless. (One of the reasons that third-party apps like Foxit PDF reader are so much smaller and tighter than Adobe Reader is that a lot of this functionality isn't supported; these third-party readers don't implement a lot of the more exotic and seldom-used parts of the PDF spec.)
