I send this form out to clients, its diffcult to know who's using what. I'm going to assume they are using the free acrobat reader. The free version should allow them to fill in the form, but but save it. The form was created in LiveCycle designer.
Do you know which PDF viewer they are using? Some have limited support for PDF forms, and some that have support for Acroforms (created with Acrobat) don't have support for XFA forms (created with LiveCycle Designer). Adobe Reader for Win/Mac should be sufficient for static or dynamic XFAs.