WordPress: Change default search query to return custom number of posts per page.

wp-search-query-custom-per-page.php

<?php

/**
 * Change the search page to return 25 posts per page in results. 
 * Add to functions.php
 */
add_filter('post_limits', 'search_postsperpage');
function search_postsperpage($limits) {
	if (is_search()) {
		global $wp_query;
		//Change 25 to whatever number you desire.
		$wp_query->query_vars['posts_per_page'] = 25;
	}
	return $limits;
}