public void SetPdfBackground(string pdfFilePath) 
{
  //重新生成的 PDF 的路徑
  string destFile = HttpContext.Current.Server.MapPath("sample.pdf");
  //create new pdf document
  FileStream stream = new FileStream(destFile, FileMode.Create, FileAccess.ReadWrite);
  PdfReader reader = new PdfReader(pdfFilePath);
  //read pdf stream 
  PdfStamper stamper = new PdfStamper(reader, stream);
  string imagePage = HttpContext.Current.Server.MapPath("../images/2012/bg2.png");
  System.Drawing.Image image = System.Drawing.Image.FromFile(imagePage);
  var img = Image.GetInstance(image, System.Drawing.Imaging.ImageFormat.Png);
  img.SetAbsolutePosition(0, 0);
  int totalPage = reader.NumberOfPages;
  for (int current = 1; current <= totalPage; current++)
  {
   var canvas = stamper.GetUnderContent(current);
   var page = stamper.GetImportedPage(reader, current);
   canvas.AddImage(img);
  }
  stamper.Close();
  reader.Close();
}