Thank you for writing.
Currently, RadPdfViewer does not support showing the pages of the documents as thumbnails. I have added this as feature request in our Public Issue Tracking System
. Feel free to add your vote for it here: http://www.telerik.com/support/pits.aspx#/public/winforms/14977
Another request you might find useful can be found here: http://www.telerik.com/support/pits.aspx#/details/Issue=14586
I hope that you find this useful.
the Telerik team