What version of FormsCentral are you using? And do you have a Paid or Free FormsCentral account.
If you are using the web version of FormsCentral and have a Free account you won't be able to save as PDF. You will need to upgrade to a paid plan (basic or plus).
If you have a paid account you can always save as PDF (from the Design Tab click on File > Save as PDF Form...)
If you are using the FormsCentral Desktop application that comes with Acrobat XI you should also be able to generate unlimitted PDF (from the Design Tab click on File > Save as PDF Form...).
Hope this helps