试试这个：

// 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 = get_post_meta( get_the_ID(), \'mcf_uppsagning-pdf\', true );
        if ( $pdf )
            $items .=  \'<li><a href="\' . wp_get_attachment_url( $pdf ) .  \'">Uppsägning</a></li>\';

    }
    return $items;
}

编辑：更改了上述代码以返回附件永久链接。