protected void btnCreatePDF_Click(object sender, EventArgs e)
{
    string pdfToModify = textBoxPdfFilePath.Text.Trim();

    // create a PDF document
    Document document = new Document(pdfToModify);

    // set the license key
    document.LicenseKey = "B4mYiJubiJiInIaYiJuZhpmahpGRkZE=";

    // get the first page the PDF document
    PdfPage firstPage = document.Pages[0];

    string logoTransImagePath = System.IO.Path.Combine(Server.MapPath("~"), @"images\evologo-100-trans.png");
    string logoOpaqueImagePath = System.IO.Path.Combine(Server.MapPath("~"), @"images\evologo-100.jpg");

    // add an opaque image stamp in the top left corner of the first page
    // and make it semitransparent when rendered in PDF
    ImageElement imageStamp = new ImageElement(1, 1, logoOpaqueImagePath);
    imageStamp.Opacity = 50;
    AddElementResult addResult = firstPage.AddElement(imageStamp);

    // add a border for the image stamp
    RectangleElement imageBorderRectangleElement = new RectangleElement(1, 1, addResult.EndPageBounds.Width,
                        addResult.EndPageBounds.Height);
    firstPage.AddElement(imageBorderRectangleElement);

    // add a template stamp to the document repeated on each document page
    // the template contains an image and a text

    System.Drawing.Image logoImg = System.Drawing.Image.FromFile(logoTransImagePath);

    // calculate the template stamp location and size
    System.Drawing.SizeF imageSizePx = logoImg.PhysicalDimension;

    float imageWidthPoints = UnitsConverter.PixelsToPoints(imageSizePx.Width);
    float imageHeightPoints = UnitsConverter.PixelsToPoints(imageSizePx.Height);

    float templateStampXLocation = (firstPage.ClientRectangle.Width - imageWidthPoints) / 2;
    float templateStampYLocation = firstPage.ClientRectangle.Height / 4;

    // the stamp size is equal to image size in points
    Template templateStamp = document.AddTemplate(new System.Drawing.RectangleF(templateStampXLocation, templateStampYLocation,
            imageWidthPoints, imageHeightPoints + 20));

    // set a semitransparent background color for template
    RectangleElement background = new RectangleElement(0, 0, templateStamp.ClientRectangle.Width, templateStamp.ClientRectangle.Height);
    background.BackColor = Color.White;
    background.Opacity = 25;
    templateStamp.AddElement(background);

    // add a true type font to the document
    System.Drawing.Font ttfFontBoldItalic = new System.Drawing.Font("Times New Roman", 10,
                System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic, System.Drawing.GraphicsUnit.Point);
    PdfFont templateStampTextFont = document.AddFont(ttfFontBoldItalic, true);

    // Add a text element to the template. You can add any other types of elements to a template like a HtmlToPdfElement.
    TextElement templateStampTextElement = new TextElement(3, 0, "This is the Stamp Text", templateStampTextFont);
    templateStampTextElement.ForeColor = System.Drawing.Color.DarkBlue;
    templateStamp.AddElement(templateStampTextElement);

    // Add an image with transparency to the template. You can add any other types of elements to a template like a HtmlToPdfElement.
    ImageElement templateStampImageElement = new ImageElement(0, 20, logoImg);
    // instruct the library to use transparency information
    templateStampImageElement.RenderTransparentImage = true;
    templateStamp.AddElement(templateStampImageElement);

    // add a border to template
    RectangleElement templateStampRectangleElement = new RectangleElement(0, 0, templateStamp.ClientRectangle.Width,
                templateStamp.ClientRectangle.Height);
    templateStamp.AddElement(templateStampRectangleElement);

    // dispose the image
    logoImg.Dispose();

    // save the document on http response stream
    try
    {
        // get the PDF document bytes
        byte[] pdfBytes = document.Save();

        // send the generated PDF document to client browser

        // get the object representing the HTTP response to browser
        HttpResponse httpResponse = HttpContext.Current.Response;

        // add the Content-Type and Content-Disposition HTTP headers
        httpResponse.AddHeader("Content-Type", "application/pdf");
        httpResponse.AddHeader("Content-Disposition", String.Format("attachment; filename=PdfStamps.pdf; size={0}", pdfBytes.Length.ToString()));

        // write the PDF document bytes as attachment to HTTP response 
        httpResponse.BinaryWrite(pdfBytes);

        // Note: it is important to end the response, otherwise the ASP.NET
        // web page will render its content to PDF document stream
        httpResponse.End();
    }
    finally
    {
        document.Close();
    }
}