pdfRenderer.Save(filename);
Process.Start(filename);

IF  (Client system web browser has possibillity of opening pdf in its window)
   Open created PDF in browser;
ELSE
   Show default download window "Would You Like to save the file...." i client browser.

 
      using(MemoryStream stream = new MemoryStream())
      {
        pDoc.Save(stream, false);
        Response.Clear();
        Response.ContentType = "application/pdf";
        Response.AddHeader("Content-Disposition", String.Format("attachment;filename={0}", pFilename));
        Response.Expires = -1;
        Response.AddHeader("content-length", stream.Length.ToString());
        Response.BinaryWrite(stream.ToArray());
        Response.Flush();
      }
      Response.End();