Adobe Acrobat 7 Professional - Manual (Page 301)

Marking comments with checkmarks
Checkmarks can be used to indicate whatever you like. For example, you can use them to
keep track of which comments you have read or which ones you want to remember.
Checkmarks are for your personal use and do not appear when others view the PDF
document unless you change the status of comments. (See
Changing the review status of
comments .)
To mark comments with checkmarks:
In the Comments List, click the checkmark box next to a comment so that the Checkmark
icon appears.