Currently, there are known issues with some images in PDF. We are making our best to fix them for the official version of RadPdfViewer in 2012 Q1. It will be great if you could send us the PDF files you have problem with so that we can analyze them. If you are a licensed developer for RadControls for Silverlight, you can open a support ticket and attach the files there.
All the best,
the Telerik team
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