How can I download a form to where I can complete it without being connected to internet?

 How can I download a form to where I can complete it without being connected to internet?

Fillable PDF copies of forms can be created:

https://www.jotform.com/help/312-JotForm-Fillable-PDF-Creator-with-All-Points

They can be filled out offline and submitted when a connection is present. 

Please note that fillable PDF forms are significantly more limited that online web forms.  Longer forms, form widgets, payments and several other features are not available in fillable PDF forms.

Can I convert a form I already have created to be able to do this?

Yes, the fillable PDF form actually needs to be generated from an already created form.  The form data will be collected and stored as if the web form was submitted. 

To generate the PDF, select the form from the My Forms section, then click the "More" button:

From that menu, click "Create PDF Form".