Unexpected cancellation charges for Adobe Acrobat Pro - never knew I signed up for annual plan

First: you posted in the stock forum, not the Acrobat or Account, Payement and Plan forum (which would habe been the correct one.).

Second: On the subscription screen, it clearly states the type of subscrition: annual payed by month, annual prepaid, monthly. You did not look correctly at your options and you did not read the terms and conditions. But yes, by error you could select the wrong, cheaper plan. But saying that it is nowhere stated is simply untrue. (For the record: I'm a user, not an Adobe employee.) You really need, next time you buy a subscription, read the terms, especially the cancellation terms. BTW: at Adobe they are a 5 minute read, and I checked this in English, French and German. So I may suppose it's the same in your language.

Third: Contact Adobe customer support and negotiate your way out of this contract. With some luck, they will waive the early termination fee. Adobe customer care can be contacted by beginning a secure chat session at https://helpx.adobe.com/contact.html?rghtup=autoOpen. Pop-up blockers need to be disabled! If the chat window fails to open, or is non-responsive, use a different device to start the interaction.
Support phone numbers may be found here: https://helpx.adobe.com/contact/phone.html. Please note that phone charges may apply.

(see also here: https://community.adobe.com/t5/creative-cloud-services/how-to-contact-adobe-support/td-p/11875703 or here
https://community.adobe.com/t5/account-payment-plan/how-to-contact-adobe-support/td-p/11843852)