Add to menu

// 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;
}