Since some PDFs do not come with their "table of contents" already made, it would be incredibly helpful to be able to create an outline that has sub-sections that can be collapsed. Such as a long textbook with multiple sections each having multiple chapters - would love to be able to easily see the section titles and then expand each as needed to see each chapter within. The textbooks that come with this already made are so nice and it would be nice to be able to do this not only with PDFs but also with my hand written notebooks.