When exporting with RtfFormatProvider (including when the users copy content), an additional \par tag is added at the end of the document.

Workaround: Process the RTF after it is generated. 

Here is an example how to achieve it when copying:

this.radRichTextBox.CommandExecuted += radRichTextBox_CommandExecuted;
...
void radRichTextBox_CommandExecuted(object sender, CommandExecutedEventArgs e)
{
    if (e.Command is CopyCommand)
    {
        DocumentPosition end = this.radRichTextBox.Document.Selection.Ranges.Last.EndPosition;
             
        RadDocument clipboardDocument = ClipboardEx.GetDocument().ToDocument();
        RtfFormatProvider provider = new RtfFormatProvider();
        string rtfString = provider.Export(clipboardDocument);
        int indexLastParOpening = rtfString.LastIndexOf("{");
        int indexLastParClosing = rtfString.IndexOf('}', indexLastParOpening);
        string newRtf = rtfString.Remove(indexLastParOpening, indexLastParClosing - indexLastParOpening + 1);
        Clipboard.SetData("Rich Text Format", newRtf);
    }
}