private void mergePdfButton_Click(object sender, EventArgs e)
{
    // Create the merge result PDF document
    Document mergeResultPdfDocument = new Document();

    // Automatically close the merged documents when the document resulted after merge is closed
    mergeResultPdfDocument.AutoCloseAppendedDocs = true;

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

    Cursor = Cursors.WaitCursor;
    string outPdfFile = @"DemoAppFiles\Output\PDF_Editor\Merge_PDF.pdf";
    try
    {                
        // The documents to merge must remain opened until the PDF document resulted after merge is saved
        // The merged documents can be automatically closed when the document resulted after merge is closed
        // if the AutoCloseAppendedDocs property of the PDF document resulted after merge is set on true like
        // in this demo applcation

        // Load the first PDF document to merge
        Document firstPdfDocumentToMerge = new Document(firstPdfFilePathTextBox.Text);
        // Merge the first PDF document
        mergeResultPdfDocument.AppendDocument(firstPdfDocumentToMerge);

        // Load the second PDF document to merge
        Document secondPdfDocumentToMerge = new Document(secondPdfFilePathTextBox.Text);
        // Merge the second PDF document
        mergeResultPdfDocument.AppendDocument(secondPdfDocumentToMerge);

        // Save the merge result PDF document in a memory buffer
        byte[] outPdfBuffer = mergeResultPdfDocument.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 PDF document resulted after merge
        // When the AutoCloseAppendedDocs property is true the merged PDF documents will also be closed
        mergeResultPdfDocument.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));
    }
}