public function joinTwoPDFs($firstPDFPath, $secondPDFPath, $outputDestination = null)
    {
        $firstPDF = new \PDFDoc($firstPDFPath);
        $firstPDF->InitSecurityHandler();

        $secondPDF = new \PDFDoc($secondPDFPath);
        $firstPDF->InsertPages(1, $secondPDF, 1, $secondPDF->GetPageCount(), \PDFDoc::e_none);
        $secondPDF->Close();

        if (!empty($outputDestination)) {
            $firstPDF->Save($outputDestination, \SDFDoc::e_remove_unused);
        } else {
            $firstPDF->Save($firstPDFPath, \SDFDoc::e_remove_unused);
        }

    }