Paid invoices appear on the Customers Record under a new Tab called "Paid Invoices". This tab only appears for Paid invoices. Proposals and Unpaid invoices can be found on the dashboard.
You can view and access any Paid invoice by searching for the customer in the CRM and pulling up their file.
You can click on the Invoice # to open the Detail Page to see all the details including internal notes associated with the paid Invoice.