@Frank I would pass the author name to addItem rather than in the public function. So around 368 of makepdf.php include something like:
$author = $item->get_author();
if ( is_object($author) ) {
$author = $author->get_name();
}

then around 370 addItem becomes:

$pdf->addItem('', $story, $date, $author);

then you can display $author somewhere in the public function addItem maybe line 76:

$html .= ''.date_i18n(get_option('date_format').' '.get_option('time_format'), $date).' | |'.$author.'';

Martin