Currently, in the web UI for Adobe Acrobat, if you've allowed comments by sharing the document via a general link (rather than sending specific invitations), the only way to disable comments at a particular point in time is to unshare the document entirely. A more useful and user-friendly approach would be similar to the proof functionality in Workfront, where even if you've shared the PDF/document with a general link (or a combo of general link and invitations), at a certain point in time, you can suspend further commenting while still allowing collaborators to access the PDF. If they access the PDF after commenting has been suspending, the UI should have elements that make it clear to them why they can no longer add comments, as they would see in Workfront. But they would still be able to see the comments, see the PDF, and download the PDF if they wish.