private void convertToPdfButton_Click(object sender, EventArgs e)
{
    // Create a HTML to PDF converter object with default settings
    HtmlToPdfConverter htmlToPdfConverter = new HtmlToPdfConverter();

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

    // Select all images from HTML page
    htmlToPdfConverter.HtmlElementsMappingOptions.HtmlElementSelectors = new string[] { "img" };

    // Exclude the original images from rendering becuase they will be replaced by an image from local file system
    htmlToPdfConverter.HiddenHtmlElementsSelectors = new string[] { "img" };

    Cursor = Cursors.WaitCursor;

    string outPdfFile = @"DemoAppFiles\Output\HTML_to_PDF\Replace_with_Higher_Quality_Images.pdf";
    Document pdfDocument = null;
    try
    {
        // Convert a HTML string with images to replace to a PDF document object
        pdfDocument = htmlToPdfConverter.ConvertUrlToPdfDocumentObject(urlTextBox.Text);

        // Replace the images selected in HTML using special attributes with images from local file system
        foreach (HtmlElementMapping imageElementInfo in htmlToPdfConverter.HtmlElementsMappingOptions.HtmlElementsMappingResult)
        {
            PdfPage imagePdfPage = imageElementInfo.PdfRectangles[0].PdfPage;
            RectangleF imageRectangle = imageElementInfo.PdfRectangles[0].Rectangle;

            ImageElement newImageElement = new ImageElement(imageRectangle.X, imageRectangle.Y, imageRectangle.Width, imageRectangle.Height, 
                            @"DemoAppFiles\Input\Images\box.jpg");
            newImageElement.EnlargeEnabled = true;
            imagePdfPage.AddElement(newImageElement);
        }

        // 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 HTML to PDF conversion failed
        MessageBox.Show(String.Format("HTML to PDF Error. {0}", ex.Message));
        return;
    }
    finally
    {
        // Close the PDF document
        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));
    }
}