<?php
function set_posts_per_page_for_post_types( $query ) {
	if ( ! is_admin() && $query->is_main_query() ) {
		if ( $query->is_search() && ! $query->is_post_type_archive() ) {
			$query->set( 'post_type', 'post' );
		}	}}
add_action( 'pre_get_posts', 'set_posts_per_page_for_post_types' );