You can Add Inline Comments to uploaded files (.xls, .ppt, .doc, etc.), in specific pages of the document.
You just need to click on the top right option Add Inline Comment and then Click anywhere in the document to make a comment. Double-clicking on the document will also let you add a comment.
A pop-up window will appear, add your notes and then click Create.
You will be able to identify how many comments have been added by looking at the top right counter or the legend located at the bottom of the document
You can either show or hide the comments in the body of the document
If you choose to see them, small paper notes will be shown and after clicking on them the comment will come up:
You can also Reply to Inline Comments:
Note: One of the disadvantages of using Inline Comments is the fact they don´t trigger any kind of notification, not even if you are the author or you are following such post.