1. New config setting added in Admin > General Settings > Invoices tab, "Scanned Invoice Required for Purchase Invoice".

When this config setting is ticked, users will not be able to create a Purchase Invoice without linking a Scanned Invoice.

2. New Permission added in Admin > Users > Select a User > Edit > User Details window > Permissions tab, Administration : "Mark Purchase Invoice as not Requiring a Scanned Invoice". 

When the config setting, "Scanned Invoices Required for Purchase Invoices" is ticked and a user has the  new permission, Mark Purchase Invoice as not Requiring a Scanned Invoice" ticked, the"Scanned Invoice not Required" checkbox is displayed in Purchase invoice details window.

When the "Scanned Invoice not Required" checkbox is ticked, it allows the Purchase invoice to be created without linking a Scanned Document.

If the user does have this Permission and the checkbox is not ticked, Purchase invoices cannot be created or Paid until a Scanned Invoice is linked.