Simply this! OpenOffice Draw is the best Offline PDF editor you could use.
You can open your pdfs easily with this free program and edit as you would any document file. Add and change text and images.
You can save it in the Open format while you edit to prevent you from losing your work and then export to pdf when you're finished. It's perfect!
I'm sure you could find many applications for this like adding notes to your pdfs (especially college lecture notes).
find it here: http://www.openoffice.org/