In Acrobat Pro, you could send a document out for shared review.

This shared review was viewable and comment-able by everyone in your shared review.

The reviewers could do things like highlight text, underline, strikethrough, and the like.

Reviewers can still do this Acrobat Pro DC, but with one exception:

The "Add-Attachment" feature.

This feature has been removed from the browser-based review used in Acrobat Pro DC.

After spending three hours on with Adobe Support, we finally found a work-around that will allow it, but trying to explain this process to people who aren't good with computers is a total nonstarter.

They just want their "little paperclip icon" back.