I agree with Stefan's advice. Here's an my article with a little more information about creating forms in Word. Please see section 4:
https://www.brandwares.com/bestpractices/2016/04/5-word-formatting-mistakes/
Since your goal is a fillable PDF, don't bother with form fields or content controls, as those will not transfer to a PDF in usable form. Instead, add the fillable fields in Acrobat.
PDFs exported from Word cannot expand in size as text is added, so you'll have to design with enough space to accommodate the maximum amount of text anyone might ever use.