Did you want to add the Print button to the "Web" form or to a PDF?
For the online form you cannot add a print button, the browser and operating system control the printing and will print whatever is viewable on the page when you print which overall does not give the desired results with FormsCentral.
For PDF - you can add a Print button to the PDF. This FAQ is about editing a FormsCentral PDF in Acrobat:
And this is a video about adding a Print button: http://www.youtube.com/watch?v=oUyvvrbT1ss