A PDF invoice will update if the invoice itself is updated or if the customer name is updated for the customer that the invoice belongs to. Anything else that's updated in a customer record, such as the customer's address only, will not cause the PDF invoice to update. Typically, it will take about 30 seconds for the PDF invoice to update after a change is made to the or invoice or the related customer name.
A PDF invoice will not update if the invoice has been marked as Billed, Paid or Won't Invoice. It also won't update if the invoice is in a problem state.