Include pages in category archive pages

gistfile1.php

<?php

/**
 * Copy paste the below code into your theme's functions.php file.
 * You could add it to a plugin instead, but it's just easier to
 * add it to your theme's functions.php. The only drawback to this
 * is if you switch themes, this functionality will stop working.
 *
 * --Alex Mills
 * Questions? Contact me: http://www.viper007bond.com/contact/
 */

// Before the normal database query runs, change its arguments to include pages in the results.
add_filter( 'pre_get_posts', 'jamesbradley_include_pages_in_category_archives' );

function jamesbradley_include_pages_in_category_archives( $query ) {
	if (
		// Only modify the main database query
		$query->is_main_query()

		// Only modify the category archives page
		&& $query->is_category()
	) {
		// Set the post type to look for to both posts and pages
		$query->set( 'post_type', array( 'post', 'page' ) );
	}
}