Preview will allow you to fill in PDF's, and then you can print this to PDF, but this does not allow you to edit text that is already in the PDF. In fact PDF is not a good format to allow for editing. A lot of things that seem like they are obviously together (including paragraphs) are actually separate elements. Sometimes each letter in a particular word is a separate object. This is all done in the name of making the output always the same.
There are a few programs out there that can sometimes edit PDF's but they are subject to a lot of mistakes, and don't always work well. I have never been able to use one of these successfully.
The way that I usually do this is to use Acrobat Reader to figure out what fonts are used in a PDF, then I import the PDF into OmniGraffle as an image, set that in a background layer, then make my edits on top of that. Then everything gets exported to a new PDF.