If you have Acrobat XI you can use the FormsCentral desktop application that comes with Acrobat to save your forms as PDF.
If you do not have Acrobat XI you will need to have a paid subscription to FormsCentral to be able to create PDF from your FormsCentral forms.
Once you create a PDF in FormsCentral you can modify it in Adobe Acrobat but you will first need to remove the Reader Extension added to the PDF by FormsCentral by opening your PDF form in Acrobat and do File > Save a Copy... You will be able to modify the copy in Acrobat. After you are done with your modifications you might want to save your new form by doing File > Save as Other... > Reder Extended PDF > Enable More Tools (includes form fill-in & save)... which will allow people with Adobe Reader to fill-in your form and save a copy on their machine.
Hope this helps