How To Watermark Pdf File Using C#?,Below Is The Code Used For Watermark On Image,Instead Of That I Wan't To Make Watermark On Pdf File.Please Suggest Solutions

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C#

Bitmap original_image = new Bitmap(FileUpload1.FileContent);
string sWaterMark = (string)s1;
int fontsize = ((original_image.Width * 2) / (sWaterMark.Length * 3));
int x = original_image.Width / 2;
int y = original_image.Height * 9 / 20;
StringFormat drawFormat = new StringFormat();
drawFormat.Alignment = StringAlignment.Center;
drawFormat.FormatFlags = StringFormatFlags.NoWrap;
Graphics graphic = Graphics.FromImage(original_image);
graphic.DrawString(sWaterMark, new Font("Verdana", fontsize, FontStyle.Bold), new SolidBrush(Color.FromArgb(80, 255, 255, 255)), x, y, drawFormat);
original_image.Save(MapPath("~/" + newfolder + "\\") + FileUpload1.FileName);
if (original_image != null) original_image.Dispose();
if (graphic != null) graphic.Dispose();