Response.OutputStream causes a postback?

[Dave]

Hi, I'm trying to download a pdf from a secure location and writ e the
response to the browser as listed below. However, the form that runs this
codes appears to postback when I encounter the Response.OutputStream???

WebClient client = new WebClient();
client.Credentials = new NetworkCredential("username", "password");
byte[] pdfBytes = client.DownloadData(strUrl);
Response.ContentType = "application/pdf";
Response.OutputStream.Write(pdfBytes, 0, pdfBytes.Length);
Response.End();