I'm using the Pdf/Mpdf class directly

use PhpOffice\PhpSpreadsheet\Writer\Pdf\Mpdf;

$writer = new Mpdf($spreadsheet);
$writer->save('05FreatureDemo.pdf');

or 

$writer = new \PhpOffice\PhpSpreadsheet\Writer\Pdf\Mpdf($spreadsheet);
$writer->save('05FreatureDemo.pdf');