To check if your customer has viewed the estimate/invoice you sent, you need to have customer portal enabled for your customer.
Once your client views the estimate/invoice, an ‘eye’ icon will be shown next to the invoice/estimate denoting that the customer has viewed the estimate/invoice and will also be seen in the estimate/invoice window.
A history notification ‘Estimate viewed in portal’ or ‘Invoice Viewed in portal’ will also be available in the estimate/invoice window notifying you.
Online accounting software
for small businesses.