function direct_free_downloads_button($button)
{
    global $product;

    if( $product->is_downloadable() AND $product->get_price() == 0 )
    {
        $files = $product->get_files();
        $files = array_keys($files);

        $download_url = home_url('?download_file='.$product->id.'&key='.$files[0].'&free=1' );

        $button = sprintf( '<a href="%s" rel="nofollow" data-product_id="%s" data-product_sku="%s" data-quantity="%s" class="button %s product_type_%s">%s</a>',
            esc_url( $download_url  ),
            esc_attr( $product->id ),
            esc_attr( $product->get_sku() ),
            esc_attr( isset( $quantity ) ? $quantity : 1 ),
            $product->is_purchasable() && $product->is_in_stock() ? '' : '',
            esc_attr( $product->product_type ),
            esc_html( 'Download' )
        );
    }
    return $button;
}
add_filter('woocommerce_loop_add_to_cart_link', 'direct_free_downloads_button', 100);



/**
 * Handles downloading of free Downloadable products
 * @return [type] [description]
 */
function download_free_product_file()
{
    $product_id    = absint( $_GET['download_file'] );
    $_product      = wc_get_product( $product_id );

    if( $_product->get_price() == 0 ) {
        WC_Download_Handler::download( $_product->get_file_download_path( filter_var($_GET['key'], FILTER_SANITIZE_STRING)  ), $product_id );
    }
}

if ( isset( $_GET['download_file'] ) && isset( $_GET['key'] ) && $_GET['free'] ) {
    add_action( 'init', 'download_free_product_file' );
}