It would be useful the possibility to link the invoices to a contract database where to store a scan of the document and to create many custom fields to classify it. Each contract should be linked to a single client or a single supplier.
This would be a radical departure from the underlying concept of the database. You can save attachments with forms, but not link forms to external documents.
My suggestion is to have a new contract form (few standard fields with the possibility to add custom fields) with a field linked to one client/supplier (dropdown field).
Each invoice (when you need it) would be linked (again with dropdown field) to the contract (with a previous preselect based on the client/supplier)
The report is essentially a list of all the invoices (id, date, amount invoiced and paid) linked to the contract (header of the report) you choose.
About the attached document, again, I expect the contract form to work like all the attachments in Manager.