Any ideas about how to prevent sharing of a pdf form except for person who paid to use it?

Only issue is it makes my reply seems out of context... Just for context: there was another reply above (and in the other thread) with a link for a faulty service that claimed to be able to do it, which was taken down.

Looks like the link to the DRM solution in China was removed from that post. 
Good Answers. 

To prevent unauthorized sharing of your PDF forms, consider using VeryPDF DRM Protector, an affordable and user-friendly solution designed to secure PDF files effectively. It allows you to lock the PDF to a specific user by email or hardware ID, disable copying and printing, and apply dynamic watermarks like the buyer’s name to discourage sharing. You can also set expiry dates or deliver the PDF through a secure web viewer, avoiding downloads altogether. Unlike traditional passwords that can be easily removed, DRM protection ensures your hard work and dynamic form features remain secure and accessible only to paying customers. You can google VeryPDF DRM Protector and try it free.

I recommend no one use this website. See my review of it here: https://community.adobe.com/t5/Acrobat/how-to-prevent-pdf-files-save-locally-or-forward-the-to-anyone/td-p/10376802

In short, the protection it offers is easily overcome and it is not clear who (or what) is behind it, that you're giving access to your sensitive files to...

There are a few things that can be done to make it difficult to share a document. You can't completely stop knowledgeable and motivated malicious users, but there are many simple (and cheap) things you can do to raise the bar to keep mostly honest users honest.

For example just password protecting a file means the user has to pass out the password, which many people are reluctant to do. You could also protect the document with a digital certificate, which is more difficult for most users to share. For an alternative type of solution, put the user's credit card number on the front front page. I knew a guy who did this to protect content.

There is also a lite security solution that uses an OCG layer to obscure the document pages until certain conditions are met. This same technique can also be used to force the user to only view the document on the desktop Reader.  Reader can run scripts that do things like contacting a web server to verify the user. But you're talking about some custom work with this solution.