it was written that the application is free. I didn't use it. I ask you to return the money. I am a father of many children.
Sorry for the inconvenience. As we can check you have opted for a premium subscription that is the reason you have got charged. Acrobat Reader mobile is a free application where you can use only a few features like View PDF, comment, fill and sign and print only but it has a premium subscription too.
Here is an FAQ on the premium subscription to understand it better: https://helpx.adobe.com/acrobat/mobile-app-faq.html
Let us know if that helps.