Hi Huebner,

You can modify the invoice document either from the frontend or by developing a custom module.

The field you specified reference is already there in the invoice, you have to add a reference in partner form in Reference

If you want to add a field of your choice in the report, Let consider the same ref field for Eg:

From frontend: activate developer mode from the setting, Now go to Settings => Technical => Reporting => Reports

search invoice and find account.report_invoice

open account.report_invoice and click on Qweb views and find report_invoice_document

Add ref field of the partner in report like you need. For Eg:

Report before and after in my Eg:

Please refer to this: Qweb Reports

"Reference" already exists, is visible on every Vendor Bill and is editable after posting.

This is the field we designed to hold Vendor References.