1. Since Q1 RadPdfViewer uses font files in order to locate the glyph outlines. This means that it requires that the used fonts must be embedded in the PDF file (according to the PDF specification, when fonts are not embedded in a PDF file, the result is implementation depended). In the cases that fonts are not embedded, RadPdfViewer tries to find the font from the system using only its font name. Sometimes this font name cannot be resolved to a valid font family. For example when you have a font name "TimesNewRoman", this is not valid "Times New Roman" font family. I guess that this is the case here. We have this issue in our to-do list, but we have not scheduled it yet.
2. Have you tried the RadRichTextBox for WPF functionality to export to PDF? Since Q1 2013 RadRichTextBox for WPF can embed used fonts in the PDF documents so they are correct according to PDF specification and in general show correctly in RadPdfViewer.
All the best,
the Telerik team