Is there a service for collective annotation of scans?

Question

PDF

Situation: there is a bunch of images (scans of abstracts), usually kindly stitched into one PDF. I would like to add notes to them (here is the definition of X, this is the theorem of Y, here is the ticket Z, there is nonsense B), and so that several people can do this (preferably at the same time, approximately like with the text in PiratePad).
I ask the community to advise a good service for this. Priority: PDF support, collaboration, free mode, enough for a session.