<?php

/**
 * Add All Custom Post Types to search
 *
 * Returns the main $query.
 *
 * @access      public
 * @since       1.0 
 * @return      $query
*/

function rc_add_cpts_to_search($query) {

	// Check to verify it's search page
	if( is_search() ) {
		// Get post types
		$post_types = get_post_types(array('public' => true, 'exclude_from_search' => false), 'objects');
		$searchable_types = array();
		// Add available post types
		if( $post_types ) {
			foreach( $post_types as $type) {
				$searchable_types[] = $type->name;
			}
		}
		$query->set( 'post_type', $searchable_types );
	}
	return $query;
}
add_action( 'pre_get_posts', 'rc_add_cpts_to_search' );
?>

Esta es la gema que nos deja Remi Corson . Incluye este código en functions.php o en algún plugin activo de WordPress

Happy coding