The first thing you can do is cancel your subscription. If subscriptions have been created through the Adobe Reader app, you can cancel your subscription here:
To cancel a subscription, go to:
Settings → Apple ID → Subscriptions
Select the subscription you would like to cancel, tap in and select "Cancel Subscription" If you are in the middle of a period, you will have access to subscriptions until it expires. After that, the subscription closes permanently.
If subscriptions are not made through Apple, then you must cancel it where you created it.
Since the subscription has been running for such a long time, I don't think you should count on getting a refund for the entire period. But you can try to get a refund for your last payment if you paid month by month.
You can read more about how to request a refund here:
Request a refund for apps or content that you bought from Apple - Apple Support