'Document awaiting download' image by the client’s message

Why is there a message saying Document Awaiting Download by the Client against a sales invoice on the Invoice Display area?

This relates to sales invoices that have been flagged for delivery via the email with link option (within the client's Client Entry), but the recipient has not yet activated and downloaded the document(s) from the link:

Any invoices which are sent as a link but not yet downloaded will revert to the Invoices Not Actioned area within the Sales Invoices menu where they can be monitored.

For more information on this method of delivery please see the following article

How do I track if an invoice has been downloaded?