Hi Shankar,
Thank you for posting your query to us.
We have support for hierachial bookmarking. This can be done by the following code snippet.
IPDFBookmark bk= BookmarksCollections.Add("Dest1",dest1);
bk.ChildNodes.Add("Dest11", dest1);
I have attached a sample based on the above code and it is available in the following link.
http://websamples.syncfusion.com/samples/PDF.Windows/67818/main.htmPlease run the sample and let me know if this satisfies you.
Regards,
Sarathi