This has been mentioned in different terms on several feature requests. I also created a request a while back but cannot find it here.
Both Wrike and Hightail have a proofing / markup feature for attachments in their system. Like in Adobe Acrobat, users are able to add comments, draw lines and containers, and other types of markup to PDFs, JPEGs and even Video attachments.
This was a feature that was listed in ClickUp's features list, but marked as planned, BUT it is no longer on that list.
Is this feature no longer planned? The fact that it was listed as PLANNED was at least 50% of the reason I had my company migrate from Wrike to ClickUp.
I hope this is indeed still in the works.