// Posts per page for custom post type Case Studies
function cpt_post_per_page( $query ) {
    if ( !is_admin() && $query->is_main_query() && is_post_type_archive( 'case-studies' ) ) {
        $query->set( 'posts_per_page', '10' );
    }
}
add_action( 'pre_get_posts', 'cpt_post_per_page' );