Similar to how you can type notes for specific slides on a powerpoint, this feature would allow you to type notes beside the document, not on the actual document. This feature could live in the sidebar of the document and be collapsable when not in use. For example, If you were using Goodnotes online or on the desktop app, you would be able to more efficiently type notes that correspond to each page without having to mess with text boxes, which often slow down workflow (espically if you are typing notes during class).