There is a similar demo in this forum thread. It keeps the titles of the documents in a list box and not a treelist view, but the approach should be quite similar. You can use HtmlDataProvider, XamlDataProvider and RtfDataProvider to bind the content of the document. There is also a TxtDataProvider, that strips the text from all formatting. You can find more information on the use of data providers in this article.
As it comes to setting the font properties of the text in the rich text box, if you set a font family and font size on the rich text box and the property DocumentInheritsDefaultStyleSettings="True", these default values will be applied to all spans that do not have explicitly set font family/size. In case you wish to change the font settings on the whole text, you can programmatically select the whole document and invoke the corresponding methods like this:
"Comic Sans MS"
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