As dagger notes, some Acrobat files are locked in such a way as to prohibit editing even with "full" Acrobat or Exchange. Some are even locked in such a way as to prohibit even copying text or graphics out of it; with that arrangement, all you can really do is print it out and OCR it. In addition, if the Acrobat file contains embedded fonts that have certain characteristics (I'm not sure *what* characteristics), text in those fonts can't be edited.
With certain PDFs that *aren't* so locked, I've personally had quite a lot of luck copying text (using the "Column Select" tool), pasting it into a word-processing or page-layout program, and proceeding from there, applying my own darn font and formatting, correcting typos, rewording sentences, adding graphics... er, well, so I'm picky.

(I'm currently working on such a project in my all-too-precious spare time.

) IIRC, the "Column Select" tool appears in Reader 5.0 as well as "full" Acrobat.
There's a brief tutorial covering certain aspects of editing with "full" Acrobat here:
http://www.aea2.k12.ia.us/tutorials/acrobat/acrobat.html
Acrobat was never meant to be a serious editing tool. For one thing, you can only edit one line of text at a time, independent of any other text... but at least you can (to take one recent example) delete ugly graphics and recenter the text on the page....

Also, the hypertext, commenting, and navigation aids you can add with Acrobat are pretty neato in the right circumstances.