Hi Rbluoin,
Thank you for your interest in Essential PDF.
Thanks for the code snippet. I was able to reproduce the issue. Currently it is not possible to insert the bookmarks into an existing document. However, as a workaround, we can copy the pages of the existing document and add them in a new PDFDocument object and then add bookmarks for this document.
Here is a sample for your reference:
Bookmarks.zip
Please take a look into it and let me know if it helps you.
Regards,
Dhivya.