To check if your customer has viewed the quote/invoice you sent, you need to have customer portal enabled for your customer.
Once your client views the quote/invoice, an eye icon will be shown next to the invoice/quote denoting that the customer has viewed the quote/invoice and will also be seen in the quote’s/invoice’s details page.
A history notification Quote viewed in portal or Invoice Viewed in portal will also be available in the Comments and History section of the quote or invoice.
Books
Online accounting software
for small businesses.