Native printing of RadDocument was implemented in Q3 2010, 2010.3.1110. However, it is always better to use the latest version of the controls, as they include the most functionality.
There is a demo showing how a RadDocument can be created from a chart, assigned to a RadRichTextBox in code-behind and printed using Silverlight's native printing in this online demo.
As for the Telerik.Windows.Documents assembly, it is included in the RadControls for Silverlight suite. If you are a licensed developer for Silverlight, you can download an installer or a hotfix of the latest version - 2011.2.920 (2011 Q2 SP1) from your account.
All the best,
the Telerik team
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