I'm trying to find a mechanism to collaboratively annotate a PDF with my class. We're reading some dense material and I'd like to have a more social and collaborative reading experience. I know I can do a collaborative google doc or ether pad doc, but I don't seem to be able to collectively comment on a PDF (even on that's been OCR-ed) in google docs. I know there are some webapps like diigo, or evernotes skitch, but I'd like to be able to do this inside of CANVAS. I'm thinking of something along the lines of how litgenius allows for collective annotation (http://genius.com/2772214/Fredric-jameson-modernism-and-imperialism/Note-the-past-tense ) Any ideas or anything obvious that I'm missing?