Comment-enabled PDFs allow authors to quickly mark up changes to their formatted book, in Adobe (Acrobat) Reader. It makes it easier for us to apply those edits, and easier equals less time equals less expensive.
Generally, authors work in a word processing program. Book designers work in costly page layout programs that require technical skills most authors don't have.
So when it comes time to make changes to a designed and formatted text, how do we bring author and designer together?
Adobe (Acrobat) Reader facilitates post-formatting edits between author and book designer, author and copy editor, author and publicist.
It's easy to use, and lets the author fine-tune a fully formatted version of their book.
Here's how it works, using Reader: