Add Water mark image to PDF using iTextsharp, C# and VB.Net in ASP.Net

Dear sandip.AP,,

please follow below example, it might be helpful

private void PdfStamp(string FileLocation, string sourceFilePath)
        {
            byte[] bytes1 = File.ReadAllBytes(@"Yourpdffilepath");
            
            var img = iTextSharp.text.Image.GetInstance(@"D:/Projects/MaterialDetails/MaterialDetails/img/Stamp.png");
            img.SetAbsolutePosition(200, 400);
            PdfContentByte waterMark;

            using (MemoryStream stream = new MemoryStream())
            {
                PdfReader reader = new PdfReader(bytes1);
                using (PdfStamper stamper = new PdfStamper(reader, stream))
                {
                    int pages = reader.NumberOfPages;
                    for (int i = 1; i <= pages; i++)
                    {
                        // Specify font and alignment of image watermark
                        waterMark = stamper.GetUnderContent(i);
                        waterMark.AddImage(img);
                    }

                }
                bytes1 = stream.ToArray();
            }
            File.WriteAllBytes(@"yourpdffilepath", bytes1);

        }