Document management system sought for small bistro

We are looking for a simple document management system (DMS) for our small bistro. It should be an archiving system for emails with invoices attached, for digitally received invoices (PDF format), and scanned receipts.

The software must meet the following requirements:

• Store emails and invoices that comply with the principles of proper management and storage of books, records, and documents in electronic form as well as data access in PDF format.
• Scan and store receipts (max. 100 invoices or receipts per month).
• Create your own structure tree (folders for emails, digital invoices, scanned invoices, subfolders for the respective invoice issuers).
• Pure storage, no pre-sorting, etc.
• A search function is not even necessary.
• Easy to use.
• The most affordable solution possible, with preference given to one-time costs rather than monthly costs or annual fees.

One to two software workstations are planned.

A WDMyCloud is available, which can also be accessed from the PC in the bistro. It would also be no problem to use an additional external or mobile 2.5-inch hard drive to work on both PCs. Or just a cloud. However, it would also be conceivable to use only the home PC. This would mean we would only need one solution for one workstation.