Does this mean that your customer has to flip between the PDF and the their reply email and write: "on Page 3, Section 4, Line 5, change XXX to YYY"?
That is a lot of work, compared to WebProof where you just place a yellow Sticky Note pointing to the error (2 clicks) and write your comment.
This Sticky Note and all others on the page are visible to all users with rights to the page.
Are you aware that even one correction typically results in 18 e-mails? Your team must spend a lot of time searching, sorting, storing and reading all those e-mails.