There is no direct support for printing rich text in the .NET Framework. To print it, you can use interop with the SendMessage API and these three messages to do your printing.
EM_SETTARGETDEVICE : specify the target device
EM_FORMATRANGE : format part of a rich edit control’s contents for a specific device
EM_DISPLAYBAND : send the output to the device
Here is a link to a MSDN article by Martin Muller that implements this interop solution.
Getting WYSIWYG Print Results from a .NET RichTextBox