Implement a method of "add comment" to Qt PDF

By "add comment" I mean something like what most of browsers do as attached screenshot shows. Select a string, and user can have an option to add comment. The commented string may be highlighted. And when cursor is over that piece of string, comment pops up.

Update: I experimented a little, and the attached "add_comment_sample" is more or less what a comment should behave. Maybe it can be a starting point to implement a better version of comment.