add_action( 'pre_get_posts', 'sk_change_testimonial_posts_per_page' );
/**
 * Change Posts Per Page for Testimonial Archive
 *
 * @author Bill Erickson
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 * @param object $query data
 *
 */
function sk_change_testimonial_posts_per_page( $query ) {

	if( $query->is_main_query() && !is_admin() && is_post_type_archive( 'testimonial' ) ) {
		$query->set( 'posts_per_page', '6' );
	}

}