webBrowser1.DocumentTextは使わずに次のとおりとする

byte[] buf = new byte[webBrowser1.DocumentStream.Length];                                 webBrowser1.DocumentStream.Read(buf, 0, (int)webBrowser1.DocumentStream.Length);

Encoding ecc = Encoding.GetEncoding(webBrowser1.Document.Encoding);

textBox1.Text =  ecc.GetString(buf);