// Filter wp_nav_menu() to add additional links and other output
add_filter('wp_nav_menu_items','add_pdf_to_menu', 10, 2);
function add_pdf_to_menu( $items, $args ) {
if( $args->theme_location == 'extra-menu') {
$pdf = 'some code i dont know i guess';
$items .= '<li>' . $pdf . '</li>';
}
return $items;
}