private void BtnOpenFile_Click(object sender, RoutedEventArgs e)
{
    OpenFileDialog dlg = new OpenFileDialog();
    dlg.Filter = "Rich Text Format (*.rtf)|*.rtf|All files (*.*)|*.*";
    if (dlg.ShowDialog() == true)
    {
        FileStream fileStream = new FileStream(dlg.FileName, FileMode.Open);
        TextRange range = new TextRange(mainRTB.Document.ContentStart, mainRTB.Document.ContentEnd);
        range.Load(fileStream, DataFormats.Rtf);
    }
}