What free software/app is there to edit pdf files on Mac OS X?
Specifically I am looking to edit an existing PDF file and re-save it in PDF format.
Have you had a look at Inkscape? It's a free, open source drawing tool. It allows importing of .PDF files and can save to .PDF as well.
Apple's built-in Preview application (free) alone sufficient to make small changes in PDF documents. When you double click on any PDF file, it opens, by default, in Preview. You can use Preview app to merge, split, mark up, annotate, and sign PDF files. Preview is the easiest solution to view, create and edit PDF documents on Mac.
With long documents or for complex editing tasks, third-party PDF editors like PDFPen or this one (cheap alternative to PDFPen) are the best bet.
I can't recommend any free solution since I don't use them, but want to second PDFPen as the best in class at any price. It looks like SamucoPDF is the least expensive app in the app store currently, but it has mixed reviews on the text editing.
If you really have more time than money, you might check out Stanza Desktop or Calibre both of which offer to accept PDF as input into file formats that are easier to edit with TextEdit or another editor you may have accessible. Once you've edited, you can print to pdf since the OS provides that to any app that can print.
PDF Studio does allow to edit text content and will work for minor text edits such as changing a few words or lines, changing font, size, color... The PDF format was not designed for Word processing and content editing is very tricky. It is usually recommended to go back to the original file (Word, Excel, etc..), to work with it and save it back to PDF. If you don't have access to the original file and need to do a lot of editing, including text reflow, wrapping, etc... you should probably bite the bullet and pay for Adobe Acrobat.
You can just use Apple's Preview application, built-in in Mac OS X.
If you need more advanced features, I recommend Skim.
There's also PDF Buddy, an online PDF editor that's really easy to use. It's a great alternative if you're not happy with Preview- especially on Snow Leopard where the Preview app is very limited.
(Disclosure: I'm a co-founder of PDF Buddy)