Hi,

First you have to set the ExportMode to Custom:

<ig:WebDocumentExporter ID="WebDocumentExporter1" runat="server" ExportMode="Custom">
</ig:WebDocumentExporter>

Then you can modify the following code to meet your requirements:

 protected void btnExportPDF_Click(object sender, EventArgs e)
 {
  Report report = new Report();
  WebDocumentExporter1.Export(WebDataGrid1, report);

  // Create whatever stream you need.
  MemoryStream stream = new MemoryStream();
  report.Publish(stream, FileFormat.PDF);

  HttpResponse response = HttpContext.Current.Response;
  response.Clear();
  response.AddHeader("content-disposition", "attachment; filename=report.pdf");
  response.ContentType = "application/pdf";
  response.Buffer = true;
  response.BinaryWrite(stream.ToArray());
  response.Flush();
  response.End();
 }