How can I change a PDF document to a fillable one?

Either the PDF has forms that use Adobe's Acrobat JavaScript and Apple's Preview does not recognize that functionality, so forms may not behave properly, or there are no forms on the PDF and you require a proper PDF Editor to add those forms to the PDF. Apple's Preview is not a PDF Editor, though there are some paid applications in the Mac App Store.

With a PDF editing app. None are free. Some say they are, but can't do anything but add annotations and highlighting, just like Preview and the Acrobat Reader can do. That's not the same as actual editing of the PDF. Others turn up in a search as an editor, but are only readers.

The ones I know of listed below. I used to use Acrobat as part of the CC suite, but now only pay for Photoshop. Switched to Master PDF Editor since it's not a subscription app. Those listed after are just ones I found but haven't used.

Adobe Acrobat Pro

Master PDF Editor

Nitro PDF Pro

PDF Expert