watermark text in all the pdf pages of existing pdf

Question

how to add watermark (text or image) in existing pdf in c#.I want the watermark in all the pages of the pdf .

I tired by itextsharp but the watermark is displaying in only last page of the pdf.

I want watermark in all pages

    public void CreateTemplate(string watermarkText, string targetFileName)

        {

            var document = new Document();

            var pdfWriter = PdfWriter.GetInstance(document, new FileStream(targetFileName, FileMode.Create));

            var font = new iTextSharp.text.Font(iTextSharp.text.Font.HELVETICA, 60, iTextSharp.text.Font.NORMAL, iTextSharp.text.Color.LIGHT_GRAY);

            document.Open();

            ColumnText.ShowTextAligned(pdfWriter.DirectContent, Element.ALIGN_CENTER, new Phrase(watermarkText, font), 300, 400, 45);

            document.Close();

        }

        public void AddTextWatermark(string sourceFilePath, string watermarkTemplatePath, string targetFilePath)

        {

            var pdfReaderSource = new PdfReader(sourceFilePath);

            var pdfStamper = new PdfStamper(pdfReaderSource, new FileStream(targetFilePath, FileMode.Create));

            var pdfReaderTemplate = new PdfReader(watermarkTemplatePath);

            var page = pdfStamper.GetImportedPage(pdfReaderTemplate, 1);

            for (var i = 0; i < pdfReaderSource.NumberOfPages; i++)

            {

                var content = pdfStamper.GetUnderContent(i + 1);

                content.AddTemplate(page, 0, 0);

            }

            pdfStamper.Close();

            pdfReaderTemplate.Close();

        }