Failed to load PDF document using mpdf 6.1

Question

I'm using codeigniter in my system and mpdf 6.1 to generate pdf. I get an error in chrome ' Failed to load PDF document ' every time when I open new tab to preview. What's wrong with that? My code in html to print:

include_once APPPATH . 'third_party/mPDF/mpdf.php';

$title= 'customer';
$html .= '<html>';
$html .= '<body>';
$html .= '<table>';
$html .= '</table>';
$html .= '</body>';
$html .= '</html>';
$mpdf = new mPDF('c', 'A4', '', '', 10, 10, 55, 20, 10, 10);
$mpdf->SetProtection(array('print'));
$mpdf->SetDisplayMode('fullpage');
$mpdf->WriteHTML($html);
$mpdf->setTitle($title);
$mpdf->Output(url_title($title.'.pdf'), 'i');

Thank you :)