How do I convert a .pdf form to a .docx form?

You can try using Adobe's free online conversion tool:

https://www.adobe.com/acrobat/online/pdf-to-word.html

I usually copy the .pdf data and paste into the blank doc. If the .pdf is very large it may take more than one copy/paste. Formatting is retained on all I've done that way.

I agree with Phile0124. I've always gotten the best results using Adobe. Note that the results you get will depend on how the PDF was created. If it was created from a scan (which is really just a photo), the results will not be as good as if it was printed to a pdf from a document.