woo_vou_gift_notification_data.php

<?php

add_filter('woo_vou_gift_notification_data', 'wpw_woo_vou_remove_downloadfile_text',10,1);

function wpw_woo_vou_remove_downloadfile_text($gift_data){
	if( isset($gift_data['voucher_link'])){
		$gift_data['voucher_link'] = str_replace('Download file', '', $gift_data['voucher_link']); // download file should be the exact that you want to remove
	}

	return $gift_data;
}