Since Acrobat is very expensive, another more budget solution might be to not work with PDF screenshots. You can always change the default screenshot format and use any simple image editor (http://guides.macrumors.com/Taking_Screenshots_in_Mac_OS_X
If that's not an option, for some PDF annotating (as you've described), you might want to try out the free Skim
or the $50 PDFpen
. If you don't want to edit the PDF per se as much as draw on it, you might also want to check out the powerful $35 GraphicConverter
or the free Inkscape
. Lastly, 10.5's Preview app can highlight and annotate PDFs, but I don't think it can do arrows or the like.