When in reading page, click in the top right corner, and you can find Annotation, Outline, Bookmark list.
It shows all the annotations (highlight, underline, strikeout, etc.) you made throughout the PDF, so it’s easier to jump between them when you need to have a quick review.
The outline will be shown if the document has one, otherwise it will be blank. Click in the outline and you can conveniently jump to a specific chapter or section.
All the bookmarks you set will show in this tab. Click on the right of each bookmark and you can rename or delete it.