Yes, if you use FormsCentral, you need to purchase a subscription to be able to save your FormsCentral form as PDF.
For pricing information, please see: https://www.acrobat.com/formscentral/en/landing/upgrade-landing2.html?accntlvl=fr
You only need to buy a subscription if you want to collect data via FormsCentral. If you want to create PDF Forms you do not need a subscription. You can generate an unlimitted number of PDF form with Forms Central for Acrobat desktop applicatin that comes with Acrobat Pro XI.
Not that the free subscription will allow you to collect responses via 1 PDF or HTML form.