iTextSharp 按页面原样导出PDF文件,页面不全问题

如题...页面导出出现一个问题
如果ASP页面太长的话,无法完全显示在PDF上的时候.超出的部分就没了只能生成一页PDF...有什么方法能够再自动生成一个PDF页面来显示
剩下的页面部分
#region pdf
        protected void btn_click(object sender, EventArgs e)
        {
            CreatPdf();
        }        private System.Drawing.Bitmap bitmap;
        private string url;
        private int w = 1440, h = 900;
        public void setBitmap()
        {
            using (WebBrowser wb = new WebBrowser())
            {
                wb.Width = w;
                wb.Height = h;
                wb.ScrollBarsEnabled = false;
                wb.Navigate(url);
                //确保页面被解析完全
                while (wb.ReadyState != WebBrowserReadyState.Complete)
                {
                    System.Windows.Forms.Application.DoEvents();
                }
                bitmap = new System.Drawing.Bitmap(w, h);
                wb.DrawToBitmap(bitmap, new System.Drawing.Rectangle(0, 0, w, h));
                wb.Dispose();
            }
        }
        private void CreatPdf()
        {
            string strChecked = Request.Form["language"];
            iTextSharp.text.Document doc = new iTextSharp.text.Document(PageSize.A4.Rotate(), 0, 0, 0, 0);//左右上下
            MemoryStream ms = new MemoryStream();
            
            try
            {
                PdfWriter writer = PdfWriter.GetInstance(doc, ms);
                writer.CloseStream = false;
                doc.Open();
                url = "此处是要导出的页面URL";
                Thread thread = new Thread(new ThreadStart(setBitmap));
                thread.SetApartmentState(ApartmentState.STA);
                thread.Start();
                while (thread.IsAlive)
                    Thread.Sleep(100);
                iTextSharp.text.Image img = iTextSharp.text.Image.GetInstance(bitmap, System.Drawing.Imaging.ImageFormat.Bmp);
                img.ScalePercent(75);//560 630
                doc.Add(img);
            }
            catch (Exception err)
            {
                throw new Exception(err.Message);
            }
            finally
            {
                doc.Close();
                ViewPdf(ms);
            }
        }
        private void ViewPdf(Stream fs)
        {
            Response.Clear();
            //中文名的话
            Response.AddHeader("Content-Disposition", "attachment;FileName=out.pdf");
            Response.AddHeader("Content-Length", fs.Length.ToString());
            Response.ContentType = "application/pdf";
            long fileLength = fs.Length;
            int size = 10240;//10K一--分块下载，10K为1块
            byte[] readData = new byte[size];
            if (size > fileLength)
                size = Convert.ToInt32(fileLength);
            long fPos = 0;
            bool isEnd = false;
            while (!isEnd)
            {
                if ((fPos + size) >= fileLength)
                {
                    size = Convert.ToInt32(fileLength - fPos);
                    isEnd = true;
                }
                readData = new byte[size];
                fs.Position = fPos;
                fs.Read(readData, 0, size);
                Response.BinaryWrite(readData);
                Response.OutputStream.Flush();
                fPos += size;
            }
            fs.Close();
            Response.OutputStream.Close();
            Response.End();
            Response.Close();
        }
        #endregion