Hi,
By override in the emails (thanks to HikaShop business ; menu System > Emails) you can add some PHP code and you can access to the $mail object.
The mail object can contains attachments though $mail->attachments.
You should also access to the $this->mailer object to add the attachments yourself.
To put your PHP code, the best is to use the "preload" email. You should find all required data (order, products, etc) in this file.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.