If you’ve enabled the Client Portal for your business in Zoho Books and configured it for your customers, they can view their transactions with you by logging into the portal. This includes viewing and accepting/declining estimates as well as viewing invoices.
The estimates and invoices can be filtered through the following steps:
Alternatively, you can simply click on the particular estimate or invoice and check if it has been viewed by the customer. A tiny Eye icon on the right corner of the estimate/invoice page accompanying the words Viewed suggests that the customer has viewed it.
Online accounting software
for small businesses.