How can I search document from web. Using C#

private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                String url = textBox1.Text;
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
                HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                StreamReader sr = new StreamReader(response.GetResponseStream());
                HtmlAgilityPack.HtmlDocument doc = new HtmlAgilityPack.HtmlDocument();
                doc.Load(sr);
                var aTags = doc.DocumentNode.SelectNodes("//a");
                int counter = 1;
                if (aTags != null)
                {
                    foreach (var aTag in aTags)
                    {
                        richTextBox1.Text += aTag.InnerHtml + "\n";
                        counter++;
                    }
                }
                sr.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Failed to retrieve related keywords." + ex);
            }