<xctk:RichTextBox  DataContext="{StaticResource EditorViewModel}" Grid.Row="1"
        Height="296" HorizontalAlignment="Center" SpellCheck.IsEnabled="True"       Margin="6,145,6,0"
        Name="richTextBoxArticleBody"  VerticalAlignment="Top" Width="962" Grid.RowSpan="2"
        BorderBrush="Silver" BorderThickness="1" AcceptsTab="True" FontFamily="Arial" FontSize="12"
        Text="{Binding PastedText, UpdateSourceTrigger=PropertyChanged}" />

private void FormatPastedTextCommandAction()
    {
        string paste = (string)Clipboard.GetData("Text");

        Clipboard.SetText(paste);

        PastedText += paste.ToString();

        Clipboard.Clear();
    }

private void FormatPastedTextCommandAction()
    {
        string paste = Clipboard.GetText();     // casting is not required if this function is used

        // Clipboard.SetText(paste);            // This line is reduntant

        PastedText += paste;                    // no need to call ToString()

        // Clipboard.Clear();                      // You should not clear the Clipboard, as the user may want to paste the data in some other window/application.
    }