I purchased the Pro subscription on one of my iPads. Do I have to buy it again on my other iPads?
If you have purchased Pro subscription on one of your iPads, you do not have to buy it again on your other iPads; as long as
a. all of those devices are signed into iCloud using the same Apple ID
b. and have 'Documents & Data' turned on in the iCloud settings app iCloud may take some time to synchronize information across all of your devices. So if you still does not work, make sure that you have internet connection and try again after a few minutes.
How do I set up iCloud to enable Perspective App to sync my Pro Subscription across all my iPads?
Setup iCloud as described in the apple documentation: apple.com/icloud/setup/ios.html. All the devices should be signed in to the same Apple ID.
Open the Settings app and make sure that the iCloud setting 'Documents & Data' is turned on.