There are issues when using custom fonts with RadRichTextBox, which we are trying to resolve. The first is that if the embedded font family is not set to the FontFamily property of a TextBox, TextBlock or RichTextBox which is in the visual tree, it does not get initialized using new FontFamily("Fonts/ChaparralPro-Regular.otf#Chaparral Pro"). There are some other cases when the behavior is inconsistent.
The only way we have found to work is to have a helper stack panel with TextBlocks using the embedded font families and when using the fonts in code-behind, reference them through the TextBlocks. It is more of a hack, but you can test how it works and decide if it is suitable and stable enough. Please find attached a demo illustrating how this approach can be implemented.
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