When exporting a document to PDF, the fonts that are used in it need to be embedded in the document to ensure proper results when the PDF is loaded in a viewer. However, the Silverlight framework imposes some limitations that do not allow accessing fonts on the client, thus embedding the needed fonts is not possible. There is a set of standard fonts that do not need to be embedded. Basically, when exporting from RadRichTextBox all fonts that are not included in the standard set fallback to one of the standard fonts. I presume this is what happens in your application as well.
Let me know if you have further comments or questions.
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