Probably require custom code for that.
Suggestion is to get back to customer and check if they can consider an easier solution with online form (new form addon recommended) and combined with a more generic pdf.
Creating a custom pdf on the fly including validation will probably cost more in the end than its worth. Only my spontaneous reflection...would be fun to code though :)
How about a regular MVC view with validation and everything, and ViewAsPdf when the user wants to print / export the form: https://github.com/webgio/Rotativa/
This won't make the PDF editable (with all the validation rules), but it could be a cheap alternative :)
Rotativa works pretty good for most sites, but last time I checked, it does not work on Azure websites due to restrictions to executing external processes. If that is the case, a cloud soultion is an easy alternative, such as html2pdfrocket.com
Here's also a blog post on creating a PDF channel for Episerver: http://joelabrahamsson.com/building-a-pdf-channel-for-episerver-7/
It's from 2012 though
I would like a form or a pagetype that the editors can edit in EPiServer when needed and the customer can fill in either online with field validations or choose to download it as a PDF. If the customer downloads it all the field validations from the HTML form should be intact so that the customer can be helped when filling it at home.
Can this be done with EPiServer? Is there already such a type or feature that I can use?