I am afraid I don't quite understand the problem you have encountered.
When it comes to getting started with RadPdfViewer - adding assembly references, declaring namespaces, etc., you can refer to this article.
As for showing a PDF file, this can be done either by the UI (which uses a command that opens a file using the OpenFileDialog) or programmatically, as described here. Note that files can be loaded by passing a URI as long as the URI specifies a path to a PDF file, that can be resolved. In Silverlight, that requires a permissive cross-domain policy file.
With regard to the download functionality - RadPdfViewer does not allow saving the PDF document.
I hope this helps.
All the best,
the Telerik team
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