I'd still go with GIMP too, given you're requirements. If you're wanting to change PDF's for presentations I would screengrab the page and then edit the image, that would be easiest.
GIMP - Downloads
Good Luck, I'm sure you're aware, but with presentations make sure you don't allow read ahead and be consistent but not too over the top with your animations to make it fun. It's all about the speaker and not the slideshow! (just my tuppence for what it's work i.e. NOTHING!)
