下载多个pdf

我正试图在我的asp.net应用程序中下载多个pdf作为附件。我已经使用pdfstamper（itextsharp）创建了一些模板和填充值。我可以填写数值，但无法下载。

private void FillForm(string path, DataTable BridgeValues, DataTable Comments, DataTable Maintenance,string Newfilename)
    {
        try
        {
            string pdfTemplate = path;
            string newFile = Newfilename;
            string Pathser = "";
            if (!System.IO.Directory.Exists(Server.MapPath(@"~/PDF/")))
            {
                System.IO.Directory.CreateDirectory(Server.MapPath(@"~/PDF/"));
            }
            if (Directory.Exists(Server.MapPath(@"~/PDF/")))
            {
               Pathser = Server.MapPath(@"~/PDF/" + Newfilename);
            }
            System.IO.MemoryStream mStream = new System.IO.MemoryStream();
            // create a new PDF reader based on the PDF template document
            PdfReader pdfReader = new PdfReader(pdfTemplate);
            PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(Pathser, FileMode.Create));
            AcroFields pdfFormFields = pdfStamper.AcroFields;
            DataColumn dc = null;
            for (int i = 0; i < BridgeValues.Columns.Count - 1; i++)
            {
                dc = BridgeValues.Columns[i];
                pdfFormFields.SetField(dc.ColumnName.ToString(),  BridgeValues.Rows[0][dc].ToString());
            }
            pdfStamper.FormFlattening = true;
            // close the pdf
            pdfStamper.Close();
            ////Response.ContentType = "application/octet-stream";
            Response.ContentType = "application/pdf";
            ////Response.AddHeader("Content-Disposition", "attachment; filename=Report.pdf");
            Response.AddHeader("Content-Disposition", "attachment; filename=" + Newfilename + "");
            ////Response.BinaryWrite(mStream.ToArray());
            Response.TransmitFile(Server.MapPath(("~/PDF/"+ Newfilename)));
            Response.Clear();
            Response.End();
        }
        catch (System.Threading.ThreadAbortException lException)
        {
            // do nothing
        }
    }

我第一次尝试创建一个pdf文件时，它成功了，但后来当我尝试下载多个文件时，却出现了异常。无法计算表达式，因为代码已优化，或者本机帧位于调用堆栈的顶部。