What is the best way to send the user a contract for signing?

Question

Email

You need a person to sign a package of documents for various bureaucrats. We want to make the task easier, because we know the full name and address of the user (we do not know the rest of the fields). That is, we can fill in a significant part of the fields of these documents. Q: What is the best way to send these documents to users? Users are the most ordinary, not IT people.
I see the following options:
- docx (or similar) in the attachment to the letter
- pdf in the attachment to the letter
- html in the attachment to the letter
- html on the site (in the letter - a link to the site)
Which format will cause the least problems for the user? Is it possible to find objective data on this topic somewhere?
Sending is initiated by the user himself, this is not a mailing list. Is there a chance that a letter with such an attachment will go to spam?

Answer 1

rtf - opens in any version of Windows out of the box (via wordpad). Just generate.
PDF the user will not be able to change -> will not be able to fill in the data.