Hello Paul,

If you wish to change anything into the Invoice PDF report you can do it by view side(from the database side).

Please find the menu path for a report of the invoice.

Settings >> Views >> search >> report_invoice_document >> Edit 

Now, you need to add your custom code either for new fields or hide fields.

Please find attached a screenshot. It will help you to find the invoice report and how to add your code.

Thanks

Regards,

Email:      odoo@aktivsoftware.com  

Skype: kalpeshmaheshwari