Include custom post types in tag archive and category archive search results in WordPress.

functions.php

<?php
//* Include custom post types in tag archive and category archive search results
add_filter( 'pre_get_posts', 'amn_get_posts' );
function amn_get_posts( $query ) {
	if ( ( is_tag() || is_category() ) && false == $query->query_vars['suppress_filters'] )
		$query->set( 'post_type', array( 'post', 'page', 'attachment', 'my_custom_post_type' ) );
	return $query;
}