RadRichTextBox's Document property is not a DependencyProperty so binding such as the one you've implemented is not possible. For the purpose of data binding documents to the editor Data Providers are exposed.
Further, in order to bind to Items controls RadDocumentBindingSource was introduced. There is also a corresponding RichTextPageViewPresenter. You can see how they work in one of RadRichTextBox's click-once demosRadBook Integration as well as in the attached sample project.
Note that the integration between the two controls has some known issues, however at this point they are not scheduled.
I hope this helps!
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