Hi,
For what I know about the plugin "PDF invoice", it allows you to send an invoice. It means that the order status must be a confirmed status (By default : confirmed, shipped).
More than that, the plugin has been developed for HikaShop but not for HikaMarket : To add invoices to the customer, so it has some specific restrictions for the targeted emails.
You can use a PDF library to generate a file and attach it to the mail.
In the preload part, you can access to the mailer, so you can add a file if you want to.
You can also see to modify the "pdf invoice" plugin in order to remove some restrictions (or change them), like the check on the order_status or the check on the order_type.
Regards,