Thank you for contacting Syncfusion support.
We tried to reproduce the reported issue from our side but, the bookmarks are exported properly to PDF as expected. We suspect that the issue might be document specific i.e., elements in the document. So, kindly share us the issue causing Word document which will be helpful for us to provide a prompt solution at the earliest.
Note: If you have any confidential data in your Word document, please replace with some dummy data and provide us the same. We just need your document to recreate the problem you face.