Client renewals should not live in one person's calendar
Email reminders before the date.
If you file licences or keep contracts for clients, the failure is a date that passed while the folder sat in Drive. A shared list with reminder days is the whole product.
What to put in
Business licences, insurance, professional registrations, software you buy on their behalf, leases if you actually own the date. Skip things the client must action without you, unless they pay you to nag them.
Reminders
Pick 30, 14 and 7 days, plus the day. Send to a mailbox that is covered when someone is off. A personal Gmail is how dates die.
Handover
When an account manager leaves, the dates must not leave with their laptop calendar. Items belong to the account, not to a person.
FAQ
Can each client be a folder or location?
Yes, use locations (or equivalent) so you can filter by client at a glance.
Do you store the PDF?
You can attach the certificate or contract when blob storage is configured. If it is not, you will get a clear error instead of a silent loss.