private void convertToPdfButton_Click(object sender, EventArgs e)
{
    // Create the PDF document where to add the HTML documents
    Document pdfDocument = new Document();

    // 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=";

    // Create a PDF page where to add the first HTML
    PdfPage firstPdfPage = pdfDocument.AddPage();

    Cursor = Cursors.WaitCursor;
    string outPdfFile = @"DemoAppFiles\Output\HTML_to_PDF\Merge_Multipe_HTML.pdf";
    try
    {
        // Create the first HTML to PDF element
        HtmlToPdfElement firstHtml = new HtmlToPdfElement(0, 0, firstUrlTextBox.Text);

        // Optionally set a delay before conversion to allow asynchonous scripts to finish
        firstHtml.ConversionDelay = 2;
                
        // Add the first HTML to PDF document
        AddElementResult firstAddResult = firstPdfPage.AddElement(firstHtml);

        PdfPage secondPdfPage = null;
        PointF secondHtmlLocation = Point.Empty;

        if (startNewPageCheckBox.Checked)
        {
            // Create a PDF page where to add the second HTML
            secondPdfPage = pdfDocument.AddPage();
            secondHtmlLocation = PointF.Empty;
        }
        else
        {
            // Add the second HTML on the PDF page where the first HTML ended
            secondPdfPage = firstAddResult.EndPdfPage;
            secondHtmlLocation = new PointF(firstAddResult.EndPageBounds.Left, firstAddResult.EndPageBounds.Bottom);
        }

        // Create the second HTML to PDF element
        HtmlToPdfElement secondHtml = new HtmlToPdfElement(secondHtmlLocation.X, secondHtmlLocation.Y, secondUrlTextBox.Text);

        // Optionally set a delay before conversion to allow asynchonous scripts to finish
        secondHtml.ConversionDelay = 2;

        // Add the second HTML to PDF document
        secondPdfPage.AddElement(secondHtml);

        // 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
        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));
    }
}