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The most common reason for the lack of all export formats is due to missing needed assembly references. The available import/export formats depend on the RadRichTextBox FormatProviders. Each FormatProvider class is located in a separate assembly reference matching its name. More information about different FormatProviders could be found at this article
Of course, it’s up to you which FormatProvider to use, consequently which document formats to be allowed for users to export to.
I hope this helps!
Let me know if you have other comments or questions.