You can't easily, unless you buy Acrobat Writer (Reader is free, Writer you have to pay for) or a PDF extraction tool. PDF is what's often referred to as an output-only format (often used for online distribution of scientific papers, so people can't crib from them as easily). It wasn't designed to be edited; one normally edits a source document and generates PDF off of it.
that isnt exactly right. you can edit with reader as long as you have 5.0 or newer and the file you give you is editable. you can download it free from adobe.com. i have reader 5.01 and it worked fine (i used it for tufts). tufts.edu at the med admissions site actually talks about adobe...so id go there and read up on what you need etc.
