Don't know why, but doing this worked... In my search.php, no problem. In content-search.php, no result. 

    while (have_posts()) :
        the_post();
        $attachment = wp_get_attachment_url(get_the_ID());
        set_query_var('document', $attachment);
        get_template_part('template-parts/content', 'search');

    endwhile;