Depends on how it was made. Sometimes they're nothing more than a giant image, thus the text isn't changeable. Other times it is. Do a search on YAHOO! or Google for PDF editor and Mac, and you should find something to do it with. I believe the pro version of Acrobat allows this also.
On a side note, keep in mind you can only modify it if you have the legal right to do so. If you don't hold the copyright to the document, then you need to get written permission before doing anything with it.