Please make sure the "Build Action" for the PDF file is set to "Resource" in the project file. Another possible reason could be that the document in not supported - as the RadPdfViewer is still in beta, there are some documents it cannot open. Can you open it with the Open File Dialog from the Open button in the built-in toolbar?
If not, you could send us the document so we can investigate it further. You will be able to attach a file if you open a support ticket through your account
Looking forward to your reply.
the Telerik team
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