Answer
The Foxit PDF Editor+ subscription purchased from Mac Apple store is the Apple's in-app purchase. You could cancel subscription as you cancel App store subscriptions for other apps on your Mac device:
1.Click the Apple icon > System Settings.
2.Click Apple ID.
3.Click Media & Purchases.
4.Next to Subscriptions, click Manage.
5.After clicking Manage, an App Store window will pop up, showing you all your subscriptions.
6. In the list of subscriptions, find the "Foxit PDF Editor+” subscription you’d like to cancel and click Edit.
7. Click Cancel Subscription, confirm the cancellation, then click Done. Note: If there’s no Cancel or Cancel Subscription button, the subscription has already been canceled.