How to sync PDF bookmarks from MacOS Preview to iOS Apple Books? Apple Books on MacOS opens PDF files in Preview. Preview can bookmark texts using CMD-D. When I open the same book with Apple Books on iOS the bookmarks are not transferred over. I have cloud sync to enabled and the books are automatically made available in iOS books. 

How do I keep my bookmarks in sync on MacOS and iOS?
There is a setting in Books.app Preferences to sync Bookmarks across devices, or not. Make sure that's on.