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function custom_projects_posts_per_page( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;
    if ( is_post_type_archive( 'project' ) || is_tax( 'project-category' )  ) {
        // Display all posts for a custom post type called 'project'
        $query->set( 'posts_per_page', -1 );
        return;
    }
}
add_action( 'pre_get_posts', 'custom_projects_posts_per_page', 1 );