To use RadRichTextEditor as a rich content viewer you have to disable the user from editing its content. This is done via the IsReadOnly property. You just have to set it to true.
this.radRichTextEditor.IsReadOnly = true;
Me.radRichTextEditor.IsReadOnly = True
In addition to the IsReadOnly property, there are several other properties that can be used to control the response of RadRichTextEditor towards the user actions against it:
Enabled - setting this property to false will disable the entire control. The user won't be able to enter any input in it and to scroll the contents.
IsSelectionEnabled - setting this property to false will disable the user to perform any selection inside the RadRichTextEditor control.
Focusable - setting this property to false will exclude the RadRichTextEditor control from the tab navigation.