<?php
$html = $divPrint;
include('mpdf.php'); // including mpdf.php
$mpdf=new mPDF();
$stylesheet = file_get_contents('pdf.css'); // external css
$mpdf->WriteHTML($stylesheet,1);
$mpdf->WriteHTML($html,2);
$mpdf->Output();
exit;
?>
1st assign your html in $html
then include mpdf.php file.