Hi Phuoc,
Thank you for your interest in Syncfusion products.
The bookmark content can be copied to another document at a specified location marked with bookmark by using bookmarksNavigator.We have prepared a sample for your requirement , kindly have a look at the sample from the below link ,let us know if it helps.
Sample:
http://help.syncfusion.com/support/samples/Diagram.Windows/F9311/Forum-9311.zipPlease let us know if you have any other questions.
Regards,
Manigandan