Can I check if my customer has viewed their invoices from the Customer Portal?
You will receive a notification in Zoho Billing when your customers view an invoice for the first time through their Customer Portal.
Also, you will be able to see an eye icon next to the invoice status in the list view of the Invoices module. You can hover over this icon to check the date and time that your customer viewed the invoice in their Customer Portal for the first time.
You can also check if customers have viewed their invoices by enabling Email Insights.