Replace text in pdf file using itextshape in c#

private void AddAnImage()
    {
        string qrfile = qrcode();

        using (var inputPdfStream = new FileStream(Server.MapPath("PDF/input.pdf"), FileMode.Open))
        using (var inputImageStream = new FileStream(qrfile, FileMode.Open))
        using (var outputPdfStream = new FileStream(Server.MapPath("PDF/output.pdf"), FileMode.Create))
        {
            PdfReader reader = new PdfReader(inputPdfStream);
            PdfStamper stamper = new PdfStamper(reader, outputPdfStream);
            PdfContentByte pdfContentByte = stamper.GetOverContent(1);
            var image = iTextSharp.text.Image.GetInstance(inputImageStream);
            image.SetAbsolutePosition(50, 75);
            pdfContentByte.AddImage(image);
            PdfContentByte canvas = stamper.GetOverContent(2);
            ColumnText.ShowTextAligned(canvas, Element.ALIGN_LEFT, new Phrase(DateTime.Now.ToShortDateString()), 0, 0, 0);
            stamper.Close();
        }

    }