Encrypt PDF created with laravel DOMPDF

gistfile1.txt

use \PDF;

use Symfony\Component\Process\InputStream;
use Symfony\Component\Process\Process;


...

        $pdf = PDF::loadHTML($html);
        $stream = $pdf->stream();

        $input = new InputStream();

        $command = 'pdftk - output - user_pw '.$password;
        $process = new Process($command);
        $process->setInput($input);
        $process->start();

        $input->write($stream->content());
        $input->close();
        $process->wait();

        $encriptedPdfContent = $process->getOutput();