Change Posts Per Page for Posts page and archives

add_action( 'pre_get_posts', 'genesiskit_change_limit_posts_per_page' );
/**
 * Change Posts Per Page for Posts page and archives.
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 * @param object $query data.
 *
 */
function genesiskit_change_limit_posts_per_page( $query ) {
    if ( $query->is_main_query() && ! is_admin() && ( is_home() || is_archive() ) ) {
        $query->set( 'posts_per_page', '8' );
    }
}