In order to properly fill out, save and submit a PDF form created using FormsCentral respondents must use Adobe Reader or Acrobat 8.0 or greater.
They can download the latest Adobe Reader from here: http://get.adobe.com/reader/
If respondents use any other PDF viewer they will likely not be able to submit the PDF form - clicking the submit button will do nothing. Additionally, field validation (required, character limits, min/max, etc) and formatting may not work correctly. Most non-Adobe PDF viewers (Apple Preview, Google Chrome PDF viewer, etc) do not fully implement the complete PDF form specification. Because of this, make sure you tell your respondents to use Adobe Reader or Acrobat to fill out the PDF form otherwise they may run into any number of problems while filling out and submitting the form.
Also note that if a FormsCentral PDF form is saved using the Apple native PDF preview application that the PDF file will be completely rewritten and will no longer work in the Adobe Reader. The form will no longer be fillable or submittable to the FormsCentral service.