Im not 100% sure, but from my experience with HighSchool newspaper, a .pdf if a "finished" product, meaning it cant be edited simply because of all the style requirements used in the program creating it -- that and the sem-large variety of programs used to create .pdfs. Sorry.
Once again, not 100% sure but there may be some sort of workaround for it. There are tools online that'll let you convert PDF to HTML for example. So you could convert it to HTML, edit, and reconvert to PDF. Possibly. This is all "theoretical".