How do I check if my client has viewed the estimate/invoice I’ve sent him?

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 accompanying the word Viewed suggests that the client has seen it.

Alternatively, go to the estimate or invoice tab under Sales on the home page of Zoho Books. Click on the drop down next to All estimates or All Invoices and select Client viewed. You can now see only the client viewed estimates/invoices.