private void stampPdfButton_Click(object sender, EventArgs e)
{
    Cursor = Cursors.WaitCursor;

    string outPdfFile = @"DemoAppFiles\Output\PDF_Editor\Stamp_PDF.pdf";
    Document pdfDocument = null;
    try
    {
        // Load the PDF document to stamp
        pdfDocument = new Document(pdfFilePathTextBox.Text);

        // Set license key received after purchase to use the converter in licensed mode
        // Leave it not set to use the converter in demo mode
        pdfDocument.LicenseKey = "fvDh8eDx4fHg4P/h8eLg/+Dj/+jo6Og=";

        // Get the stamp width and height
        float stampWidth = float.Parse(stampWidthTextBox.Text);
        float stampHeight = float.Parse(stampHeightTextBox.Text);

        // Center the stamp at the top of PDF page
        float stampXLocation = (pdfDocument.Pages[0].ClientRectangle.Width - stampWidth) / 2;
        float stampYLocation = 0;

        RectangleF stampRectangle = new RectangleF(stampXLocation, stampYLocation, stampWidth, stampHeight);

        // Create the stamp template to be repeated in each PDF page
        Template stampTemplate = pdfDocument.AddTemplate(stampRectangle);

        // Create the HTML element to add in stamp template
        HtmlToPdfElement stampHtmlElement = new HtmlToPdfElement(htmlStringTextBox.Text, baseUrlTextBox.Text);

        // Set the HTML viewer width for the HTML added in stamp
        stampHtmlElement.HtmlViewerWidth = 600;
        // Fit the HTML content in stamp template
        stampHtmlElement.FitWidth = true;
        stampHtmlElement.FitHeight = true;

        // Add HTML to stamp template
        stampTemplate.AddElement(stampHtmlElement);

        // Save the PDF document in a memory buffer
        byte[] outPdfBuffer = pdfDocument.Save();

        // Write the memory buffer in a PDF file
        System.IO.File.WriteAllBytes(outPdfFile, outPdfBuffer);
    }
    catch (Exception ex)
    {
        // The PDF creation failed
        MessageBox.Show(String.Format("Create PDF Document Error. {0}", ex.Message));
        return;
    }
    finally
    {
        // Close the document to stamp
        if (pdfDocument != null)
            pdfDocument.Close();

        Cursor = Cursors.Arrow;
    }

    // Open the created PDF document in default PDF viewer
    try
    {
        System.Diagnostics.Process.Start(outPdfFile);
    }
    catch (Exception ex)
    {
        MessageBox.Show(String.Format("Cannot open created PDF file '{0}'. {1}", outPdfFile, ex.Message));
    }
}