/// <summary>
  /// 调用之前请确认当前调用文件夹和文件是否正确
  /// </summary>
  /// <param name="filePath"></param>
  /// <param name="toPath"></param>
  private void ConvertPDFToPDF(string filePath,string toPath)
  {
   PdfReader reader = new PdfReader(filePath);

   int n = reader.NumberOfPages;
   

   PdfStamper stamp = new PdfStamper(reader, new FileStream(toPath,FileMode.Create));
   
   int i = 0;
   PdfContentByte under;
   
   Image mm = Image.GetInstance("mm.jpg");
   mm.SetAbsolutePosition(470,780);
   mm.ScaleAbsolute(120,30);
   
   
   while (i < n)
   {
    i++;
    under=stamp.GetOverContent(i);

    //图片
    under.AddImage(mm);

   }

   stamp.Close();
   reader.Close();
  }