$path = 'file.pdf';
$mpdf = new \Mpdf\Mpdf(['format' => 'A4', 'orientation' => 'P', 'default_font' => 'arial']);
$mpdf->SetSourceFile($path);
$tplId = $mpdf->ImportPage(1);
$mpdf->UseTemplate($tplId);
$mpdf->Output(str_replace(['.pdf'], ['_sign.pdf'], $path), 'F');