Is there a way to make a PDF document fillable that has already been created, using either a previously installed app or a free app on a Mac?
I have Pages, Word 2021, Adobe Acrobat installed, but they all seem to want to charge additional for this option.
I am trying to fill out an application that is a PDF. There are quite a few fields that I need to fill out and my handwriting is terrible.
Two different things here: making a PDF form that others can fill out, and filling out a form someone else has made.
Your initial question seems to be about making PDF forms. For that, Adobe Acrobat is what you need. Perhaps you have Adobe Reader instead? Either way, Acrobat is the main app that is used. Pages can't do it. There may be some other third-party apps that can, but I don't know of any offhand. Probably nothing free.
Your "Why" section seems to suggest the opposite, that you are looking to fill out a form that someone else has made. You can do this with just Preview on your Mac. That's the default PDF reader, though if you have installed Adobe things, then double-clicking may automatically open the Adobe apps. Open it in Preview instead then you can fill out any form fields that are in the PDF.
If the form doesn't have any pre-made fillable fields, you can always use markup tools to put any text you want any any location. See https://macmost.com/the-mac-preview-pdf-markup-tools.html
I was trying to fill out a form that someone else made. I was unable to make it work using preview. I thought perhaps it was because it was scanned document, but I even downloaded it from there companies website and then exported it to the desktop as a PDF, it still does not allow me to fill it in.
Found a work around. Opened up the PDF in Adobe Acrobat Reader, then went to "fill and sign" under tools and it worked.