> and mark up with highlights and notes, has been a game-changer.

What are you using for that? In my experience annotation support is very spotty, not user-friendly and not cross-platform, so I rarely use it for collaboration. Even fill-in forms one gets for reimbursements often seem badly done with too small or too large boxes or missing boxes. Thus, pdf seems fine for read-only, but making changes afterwards is not a good experience.