Interactive PDFs use the PDF reader plugin associated with the browser. Properly configured PDFs can be commented using Adobe Reader XI however when viewed within a container field the options to add a new comment are greyed out so I don't think it is an option.
A possible, although maybe not ideal, solution would be a table of related "PDF Comment" records. They could be displayed in a portal beside the container. Users could add comment records there. Of they wouldn't be on the actually PDF so it would require the user to be very clear about what they are commenting on.
I've Adobe Professional and it never worked. The commenting tools are disabled.
I think collaborating on same document at the same time by many users is asking for a lot from filemaker.
You may be able to achieve a satisfactory result by exporting the pdf using TemporaryPath, user opens the pdf in AcrobatPro, comments, then reloads the modified version back into the container. You would have to lock the FM record while this was happening to avoid uses tripping over one another. I haven't tried this, mind you, but in theory it should work.