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 client viewed estimates and invoices can be filtered for separate viewing,
Alternatively, you can simply click on the particular estimate or invoice and check if it has been viewed by the client. A tiny eye icon on the right corner of the estimate/invoice page accompanying the words Viewed suggests that the client has viewed it.