using (fs = new FileStream("C:\Temp\doc.rtf", FileMode.Create, FileAccess.Write))
 {
  byte[] b1 = new byte[(dr.GetBytes(3, 0, null, 0, int.MaxValue))];
  dr.GetBytes(3, 0, b1, 0, b1.Length);
  fs.Write(b1, 0, b1.Length);
 }
FileStream fs1 = new FileStream("C:\Temp\doc.rtf", FileMode.Open); 
sr = new StreamReader(fs1);
RichTextBox1.Rtf = sr.ReadToEnd();
sr.Close(); fs1.Close();

 byte[] b = new byte[(dr.GetBytes(0, 0, null, 0, int.MaxValue))];
 dr.GetBytes(0, 0, b1, 0, b1.Length);
 richTextBox1.Rtf=System.Text.Encoding.ASCII.GetString(b);

public static byte[] StrToByteArray(string str)
{
    System.Text.ASCIIEncoding  encoding=new System.Text.ASCIIEncoding();
    return encoding.GetBytes(str);
}