pdf2oo converts pdf to odf. It creates an image for each page, allowing to overlay drawings and text and export it back to pdf. Handy for rewiews of scientific papers. The closest thing to a free pdf editor, "almost" an openoffice pdf import filter.
You can even add animations to your beamer presentations!
You can associate pdf2oo with your pdf files
The graphical user interface right after opening a pdf
Here is how a paper looks like just after import in OO
You can overlay text and graphics