Everybody who is interested in this item, could also be interested in this: http://feedback.telerik.com/Project/184/Feedback/Details/190792-support-digital-signature-pdf-in-exported-pdf

So go and vote for it!

And there are a lot of people since 2009 waiting for digital signatures!

http://www.telerik.com/forums/digital-signature-in-exported-pdf

But you can use an alternate solution until Telerik reacts: If you export your PDF to a memorystream you can sign it with this library: http://www.signfiles.com/signature-library/

Everbody who wants this feature should also subscribe and vote for: http://feedback.telerik.com/Project/143/Feedback/Details/151016-add-digital-signature-to-pdf-on-exporting-in-radrichtextbox