Adding comments to a working document can be a great help. For instance, I just finished editing a 166 page course and as I went through it I came up with a number of questions for the author. I could have made a list, but that would have meant switching out of the document I was working on and opening a new one for notes. It was much easier to just add notes as I went.
The notes in the text have the added advantage of letting the author know exactly where the problem exists.
This works in Pages and Numbers, but I have only ever used it in Pages. How does it work?
Whenever you come to a place that needs a comment, hold your cursor right at that spot and click on the comments icon at the top of the page. (red arrow). The yellow comment will open in a new, left hand field and you can input as much or as little as needed. The comment remains until the issue is solved and you click the little x to delete it.
This little feature can keep you out of trouble.