Hi,
You can modify the plugin "attachinvoice" in order to authorize new mail names, by replacing this line
if(empty($data->mail_name) || empty($data->data->order_status) || !in_array($data->mail_name,array('order_status_notification','order_creation_notification')) || $data->data->order_status != $confirmed) return;
The plugin attach the PDF in the emails "order_status_notification" and "order_creation_notification", so it does not attach in the HikaMarket emails or in the admin emails.
But by adding the email name in the list, you can change that.
Regards,