There is a known issue that is connected with resource files that are embedded in the Telerik.Windows.Documents.Fixed.dll. In some cases, RadPdfViewer cannot access those resources and throws InvalidOperationException - "Cannot access the resource file.". You can check if this is your case if you try to import your document using directly PdfFormatProvider.Import() method instead of using the PdfDocumentSource. If this is your case, the InvalidOperationException will be thrown when you call the Import() method.
We are looking forward to your reply.
the Telerik team