How to optimize this code.it take lot of time to export pdf using itextsharp DLL

   Document doc = new Document(PageSize.A4, 10, 10, 40, 35);
//   MemoryStream ms = new MemoryStream();

   string path = System.Web.Configuration.WebConfigurationManager.AppSettings["ExportFilePath"];

   String FilePath = path + "Test" + _exportCSV + ".pdf";
   if (File.Exists(FilePath))
   {
       File.Delete(FilePath);
   }
   PdfWriter writer = PdfWriter.GetInstance(doc, new FileStream(FilePath, FileMode.Create));

   writer.InitialLeading = 0;
   DataTable dt1 = ds.Tables[0];
   DataTable dt2 = ds.Tables[1];
   DataTable dt3 = ds.Tables[2];

   DataTable newDt2 = new DataTable();
   DataTable newDt3 = new DataTable();

   UserSession OUser = (UserSession)HttpContext.Current.Session["User"];
   string wtUnit = OUser.UserWeightAbbriviation;
   string volUnit = OUser.UserVolumeAbbriviation;
   string userRevenueUnit = (OUser.UserCurrencyID == null ? OUser.BUCurrencyAbbriviation : OUser.UserCurrencyAbbriviation);

   var normalFont = FontFactory.GetFont(FontFactory.TIMES, 4);
   var boldFont = FontFactory.GetFont(FontFactory.TIMES_BOLD, 4);

   doc.Open();

   PdfPTable table = new PdfPTable(dt1.Columns.Count - 1) { WidthPercentage = 100 };
   table.Complete = false;
   table.SplitLate = false;
   for (int j = 0; j < dt1.Columns.Count; j++)
   {
       if (dt1.Columns[j].ColumnName == "Req_Id")
           continue;
       if (dt1.Columns[j].ColumnName == "Wt")
           table.AddCell(new Phrase((dt1.Columns[j].ColumnName).Replace("_", " ") + " (" + wtUnit + ")", boldFont));
       else if (dt1.Columns[j].ColumnName == "Vol")
           table.AddCell(new Phrase((dt1.Columns[j].ColumnName).Replace("_", " ") + " (" + volUnit + ")", boldFont));
       else if (dt1.Columns[j].ColumnName == "Rate" || dt1.Columns[j].ColumnName == "Bid_Price")
           table.AddCell(new Phrase((dt1.Columns[j].ColumnName).Replace("_", " ") + " (" + userRevenueUnit + ") /(" + OUser.UserWeightAbbriviation + ")", boldFont));
       else if (dt1.Columns[j].ColumnName == "ULD_No")
           table.AddCell(new Phrase(dt1.Columns[j].ColumnName.Replace("_No", " #"), boldFont));
       else if (dt1.Columns[j].ColumnName == "Release_Per")
           table.AddCell(new Phrase(dt1.Columns[j].ColumnName.Replace("_Per", " (%)"), boldFont));
       else
           table.AddCell(new Phrase(dt1.Columns[j].ColumnName.Replace("_", " "), boldFont));
   }

   if (dt1.Rows.Count >= 1 && !String.IsNullOrEmpty(Convert.ToString(dt1.Rows[0][0])))
   {

       for (int i = 0; i < dt1.Rows.Count; i++)
       {
           for (int j = 0; j < dt1.Columns.Count; j++)
           {
               if (j == 0)
                   continue;
               table.AddCell(new Phrase(dt1.Rows[i][j].ToString(), normalFont));
           }
           var rows = (from bb in dt2.AsEnumerable()
                       where (bb.Field<string>("Request_ID") == Convert.ToString(dt1.Rows[i]["Req_Id"]))
                       select bb);

           int colDiff = (dt1.Columns.Count - dt2.Columns.Count) + 2;

           for (int k = 0; k < dt2.Columns.Count; k++)
           {
               PdfPCell cell1 = null;
               if (dt2.Columns[k].ColumnName == "Request_ID" || dt2.Columns[k].ColumnName == "Route_Header_ID" || dt2.Columns[k].ColumnName == "Path_ID")
                   continue;
               if (dt2.Columns[k].ColumnName == "Rec_Wt_Model" || dt2.Columns[k].ColumnName == "Rec_Wt_User")
                   cell1 = new PdfPCell(new Phrase(dt2.Columns[k].ColumnName.Replace("_", " ") + " (" + wtUnit + ")", boldFont));
               else if (dt2.Columns[k].ColumnName == "Rec_Vol_Model" || dt2.Columns[k].ColumnName == "Rec_Vol_User")
                   cell1 = new PdfPCell(new Phrase(dt2.Columns[k].ColumnName.Replace("_", " ") + " (" + volUnit + ")", boldFont));
               else if (dt2.Columns[k].ColumnName == "Route_Cost")
                   cell1 = new PdfPCell(new Phrase(dt2.Columns[k].ColumnName.Replace("_", " ") + " (" + userRevenueUnit + ") /(" + OUser.UserWeightAbbriviation + ")", boldFont));
               else
                   cell1 = new PdfPCell(new Phrase(dt2.Columns[k].ColumnName.Replace("_", " "), boldFont));
               table.AddCell(cell1);
               if ((k == dt2.Columns.Count - 1) && colDiff > 0)
               {
                   cell1 = new PdfPCell(new Phrase(""));
                   cell1.Colspan = colDiff;
                   table.AddCell(cell1);
               }
           }


           if (rows.Count() > 0)
           {
               newDt2 = rows.CopyToDataTable();

               for (int i1 = 0; i1 < newDt2.Rows.Count; i1++)
               {
                   for (int j1 = 0; j1 < newDt2.Columns.Count; j1++)
                   {
                       if (j1 == 0 || j1 == 1 || j1 == 2)
                           continue;
                       PdfPCell cell1 = new PdfPCell(new Phrase(newDt2.Rows[i1][j1].ToString(), normalFont));
                       table.AddCell(cell1);

                       if ((j1 == newDt2.Columns.Count - 1) && colDiff > 0)
                       {
                           PdfPCell cell2 = null;
                           var rows3 = (from bb in dt3.AsEnumerable()
                                        where (bb.Field<string>("Request_ID") == Convert.ToString(newDt2.Rows[i1]["Request_ID"])
                                        && bb.Field<string>("Route_Header_ID") == Convert.ToString(newDt2.Rows[i1]["Route_Header_ID"]))
                                        select bb);
                           var totalRows = 0;
                           if (rows3.Count() > 0)
                           {
                                totalRows = rows3.Count() + 1;
                           }
                           cell2 = new PdfPCell(new Phrase(""));
                           cell2.Colspan = colDiff;
                           if (i1 == 0)
                           {
                               totalRows = totalRows + 1;//start end date hedaer
                           }

                           cell2.Rowspan = totalRows + 1;
                           table.AddCell(cell2);
                       }
                   }



                   var rows1 = (from bb in dt3.AsEnumerable()
                                where (bb.Field<string>("Request_ID") == Convert.ToString(newDt2.Rows[i1]["Request_ID"])
                                        && bb.Field<string>("Route_Header_ID") == Convert.ToString(newDt2.Rows[i1]["Route_Header_ID"]))
                                select bb);

                   int colDiff1 = (dt2.Columns.Count - dt3.Columns.Count);

                   for (int k = 0; k < dt3.Columns.Count; k++)
                   {
                       PdfPCell cell1 = null;
                       if (dt3.Columns[k].ColumnName == "Request_ID" || dt3.Columns[k].ColumnName == "Route_Header_ID" || dt3.Columns[k].ColumnName == "Path_ID")
                           continue;
                       if (dt3.Columns[k].ColumnName == "Flt_No")
                           cell1 = new PdfPCell(new Phrase(dt3.Columns[k].ColumnName.Replace("_No", " #"), boldFont));
                       else
                           cell1 = new PdfPCell(new Phrase(dt3.Columns[k].ColumnName.Replace("_", " "), boldFont));
                       table.AddCell(cell1);

                       if ((k == dt3.Columns.Count - 1) && colDiff1 > 0)
                       {
                           cell1 = new PdfPCell(new Phrase(""));
                           cell1.Colspan = colDiff1;
                           cell1.Rowspan = (rows1.Count()) + 1;
                           table.AddCell(cell1);
                       }
                   }
                   if (rows1.Count() > 0)
                   {
                       newDt3 = rows1.CopyToDataTable();
                       for (int i2 = 0; i2 < newDt3.Rows.Count; i2++)
                       {
                           int colLeg = 0;
                           for (int j2 = 0; j2 < newDt3.Columns.Count; j2++)
                           {
                               if (j2 == 0 || j2 == 1 || j2 == 2)
                                   continue;
                               PdfPCell cell1 = new PdfPCell(new Phrase(newDt3.Rows[i2][j2].ToString(), normalFont));
                               table.AddCell(cell1);
                               colLeg = colLeg + 1;
                           }
                       }
                   }
                   else
                   {
                       string rowData = "No child records to display.";
                       PdfPCell NoRowcell = new PdfPCell(new Phrase(rowData, normalFont));
                       NoRowcell.Colspan = dt2.Columns.Count - 1;
                       table.AddCell(NoRowcell);
                   }
                   doc.Add(table);
                   table.DeleteBodyRows();
               }
           }
           else
           {
               string rowData = "No child records to display.";
               PdfPCell Rowcell1 = new PdfPCell(new Phrase(rowData, normalFont));
               Rowcell1.Colspan = dt1.Columns.Count - 1;
               table.AddCell(Rowcell1);
           }

           doc.Add(table);
           table.DeleteBodyRows();
       }
   }
   iTextSharp.text.FontFactory.FontImp = new iTextSharp.text.FontFactoryImp();
   GC.Collect(GC.MaxGeneration);
   table.Complete = true;
   doc.Add(table);
   doc.Close();

   var file = new System.IO.FileInfo(FilePath);
   Response.Clear();
   Response.Buffer = true;
   Response.AddHeader("Content-Disposition", "attachment; filename=" + _exportCSV + ".pdf");
   Response.AddHeader("Content-Length", file.Length.ToString(CultureInfo.InvariantCulture));
   Response.ContentType = "application/pdf";
   Response.WriteFile(file.FullName);
   HttpContext.Current.Response.Flush(); // Sends all currently buffered output to the client.
   HttpContext.Current.Response.SuppressContent = true;  // Gets or sets a value indicating whether to send HTTP content to the client.
   HttpContext.Current.ApplicationInstance.CompleteRequest(); // Causes ASP.NET to bypass all events and filtering in the HTTP pipeline chain of ex
   Response.Close();
   if (File.Exists(FilePath))
   {
       File.Delete(FilePath);
   }