Я нашел и соединил следующее на основе этого сайта :
object missing = System.Reflection.Missing.Value;
object Visible = true;
object start1 = 0;
object end1 = 0;
Word.Document adoc = WordApp.Documents.Add(ref missing, ref missing, ref missing, ref missing);
Word.Range rng = adoc.Range(ref start1, ref missing);
try
{
Clipboard.SetText(richTextBox.Text, TextDataFormat.Rtf);
WordApp.Selection.Paste();
string path = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
object filename = path + @"\MyWord.doc";
adoc.SaveAs(ref filename, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
WordApp.Visible = true;
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}