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();}