Starting from HSPcomplete 3.2 you can edit action on notification at
Top > Configuration Director > Event Manager > Events > EVENT_NAME > Actions > Send message > "select template" > Next
On the page find section "Send a printable form as a PDF-document attached to this message" and enable "Invoice details".
Remember to verify valid PDF Generator is configured in PBA-S:
Top > Configuration Director > Miscellaneous Settings > PDF Generator Setup
Please refer to the following documentation article for more information: Plugging in Different PDF Generator