I had to do something similar for some legal stuff myself. So, I found this really easy guide on a site called https://setapp.com/how-to/export-iphone-messages-to-pdf . It basically explains how to save your iPhone messages as a PDF. You need a software called AnyTrans for iOS. Just hook up your iPhone to your computer, choose the messages you want to keep and export them as a PDF. If you don't have a computer handy, you can also take screenshots of your messages and then convert those into a PDF. To open the PDF, you just need any standard PDF reader which you probably already have on your phone or computer. It all sounds a bit techy but trust me, it's simpler than it sounds. Give it a shot and let me know if you get stuck anywhere. Good luck with your paperwork!