$pdf = new FPDI();

    $pdf_path = $_SERVER["DOCUMENT_ROOT"] . "\files\" . md5(uniqid("")) . ".pdf";
    $path_assinado = $_SERVER["DOCUMENT_ROOT"] . "\files\" . md5(uniqid("")) . ".pdf";

    file_put_contents($pdf_path, $pdfDocument);

    $pageCount = $pdf->setSourceFile($pdf_path);
    for ($i = 1; $i <= $pageCount; $i++) {
        $pdf->AddPage();
        $page = $pdf->importPage($i);
        $pdf->useTemplate($page, 0, 0);
    }

    $info = [
        'Name'        => $certificado->getCompanyName(),
        'Date'    => date("Y.m.d H:i:s"),
        'Reason'      => 'Assinatura',
        'ContactInfo' => 'contact',
    ];
    $pdf->setSignature($certificado->__toString(), $certificado->privateKey, $senha, '', 1, $info);


    $pdf->Output($path_assinado, "F");

    return file_get_contents($path_assinado);