I think this can be caused by missing fonts in your PDF file. When the font is not preset in the document, RadPdfViewer tries to find a substitute and the result is unpredictable. Note that according to PDF specification, all fonts should be embedded in the document. Otherwise the result is implementation dependent.
You could try to send us your document in order to further investigate the issue, but if you have control over how this document is created, the best way to fix it is to embed all used fonts.
Hope this answers your question.
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