You have all the references you need. A possible cause of the problem can be mismatched assembly references, e.g. the version of Telerik.Windows.Documents.FormatProviders.Rtf being different than that of Telerik.Windows.Documents. If your project is WPF 4.0, make sure you are using the assemblies for WPF 4.0 and the version of all assemblies is the same.
If this information doesn't help you, please prepare a simple demo project illustrating the problem and get back to us so we can investigate it further.
the Telerik team
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