add_filter('post_grid_layout_element_author_link_permalink', 'post_grid_layout_element_author_link_permalink_20200614', 10, 2);

function post_grid_layout_element_author_link_permalink_20200614($post_link, $args){
$post_id = isset($args['post_id']) ? $args['post_id'] : '';
$download_url = get_field( "download_url", $post_id );
$post_link = !empty($download_url) ? $download_url : $post_link;
return $post_link;
}