Not sure if you have access to an official copy of Adobe Acrobat; or one
of the applications the author used, because that is a most certain means.
Even then, some forms cannot be altered; permissions to fill blanks, aside.
support.apple.com/kb/PH20226?locale=en_US
Sep 27, 2016 - Fill out a PDF form. Open the PDF form in Preview. Click a field or a line
in the form. A text annotation box automatically appears. Type your text in the field.
If a PDF form is static, you can still fill it out without having to print and do so by hand.
... Fill PDF forms (Acrobat 8.0 on Windows or Mac OS) ... Support Knowledgebase
Fill PDF forms ... settings to Allowed before you can fill the PDF form on ...
adobe.com/support/techdocs/333232.html
> Filling in PDF forms. Filling in PDF forms. Search. ... see Fill out your PDF form.
Save form, ... or Acrobat DC/Acrobat Reader DC > Preferences (Mac OS).
helpx.adobe.com/acrobat/using/filling-pdf-forms.ht
Other ideas via links appear in this search result; untested as-is:
https://duckduckgo.com/?q=how+to+fill+pdf+forms+on+mac&ia=web
Not sure if your macOS Help Viewer could find a suitable answer to doing this
with applications included in the OS; or if you'd be expected to buy an App.
In any event...
Good luck & happy trails! 🙂