Bookmark UI is too narrow
1. Its balloon UI is too small, long bookmark names does not fit into the balloon UI's width.
2. It requires clicking into several "folders" in order to access a nested bookmark entry. It'd be better to have them expanded by default (which would require the point 1 to be addressed).
Personally I am happy with a UI for browsing bookmarks of pdf as implemented in an app called "kybook 3"