If Parent or Current Page Is, Then Statement?

by

Use get_post_ancestors():

    // get page ancestors
    $parents = get_post_ancestors( $post->ID );

    // id of parent page you want to find
    $parent_id = 123;

    // check of $parent_id
    if ( $post->ID == $parent_id || in_array( $parent_id, $parents ) ) {
        echo '<a href="#">Link</a>';
    }