Add custom post types to pre_get_posts for main loop WP

cpt-to-main-loop.php

add_filter( 'pre_get_posts', 'my_get_posts' );

function my_get_posts( $query ) {

    if ( is_category() && $query->is_main_query() )
        $query->set( 'post_type', array( 'post', 'page', 'fonts' ) );

    return $query;
}