给已经存在的PDF文件加水印

  static public bool WatermarkPDF(string SourcePdfPath, string OutputPdfPath, string WatermarkPath, int positionX, int positionY, int WatermarkHeight, int WatermarkWidth, out string msg)
        {
            try
            {
                PdfReader reader = new PdfReader(SourcePdfPath);
                PdfStamper stamp = new PdfStamper(reader, new FileStream(OutputPdfPath, FileMode.Create));
                int n = reader.NumberOfPages;
                int i = 0;
                PdfContentByte under;
                iTextSharp.text.Image im = iTextSharp.text.Image.GetInstance(WatermarkPath);
                im.SetAbsolutePosition(positionX, positionY);
                im.ScaleAbsolute(WatermarkWidth, WatermarkHeight);

                while (i < n)
                {
                    i++;
                    under = stamp.GetUnderContent(i);
                    under.AddImage(im, true);

                }
                stamp.Close();
                reader.Close();
            }
            catch (Exception ex)
            {
                msg = ex.Message;
                return false;
            }
            msg = "加水印成功！";
            return true;
        }

调用方法

WatermarkPDF("N.pdf", "N1.pdf", "Signer.JPG", 200, 200, 100, 200, out msg)