A good reminder names the invoice number, amount and due date, links to the payment page or attaches the invoice, and asks politely whether anything is missing. A click on the payment link is a stronger signal than an open, but neither proves that the person read or agreed to anything.
Tracking of invoice PDFs page by page is a preview in HawkOpen: the server accepts files only after an external malware scan is switched on. Until then, attach the invoice in your mail app or link to it.