I can't download PDF FORM(Mac).

If that PDF document uses Adobe Acrobat JavaScript for certain form controls, or the PDF was created in Adobe LiveCycle Designer on Windows, Apple's Preview may not have the smarts to open the PDF or give you access to those form fields.

I recommend that you download and install the current Adobe Acrobat Reader DC (not the Acrobat Pro DC trial) as only Adobe can keep their PDF reader current with evolving PDF standards. Once you have the Adobe Reader product installed, right-click on that PDF and choose Open With > Adobe Acrobat Reader DC.