How do i save a text message chain to a document?

That, or the explanation that you got from Carl. There are some other 3rd party apps that can copy for archive, but many of them do not do such a good job at identifying the writer of the particular message. For a true picture, taking screenshots and then putting them together is probably the easiest.

A couple of the 3rd party programs that you load on a computer are TouchCopy, which works on the Windows platform, or PhoneView, which works on the Mac. I've used TouchCopy, they have a free version that will do some things depending on how much data, or there is a paid version, which I have used. I was able to archive a number of voicemail messages and texts for a court case.

You would have to copy all of the data from the chain and then paste it to a document. There is no native way to change a text to a document.

Best you can do with native options is making screenshots of the conversation, then move them to computer and paste in a Word doc. You can check iMazing, it allows exporting messages to PDF/TXT/Excel, but it's paid (guide if needed).