Add PDF bookmarks

[Kristine1975]

The v1.10 pdf doesn't seem to have PDF bookmarks, that a PDF reader can display to provide an outline of the book for quick navigation. I think this would be useful to have.

[ActoryOu]

Hello, @Kristine1975,
Thank you for bringing this to our attention! We will be looking further into this issue as soon as we can, thank you for your patience!