Can't figure out a way for exporting text messages from iphone to pdf

The reason you can't find it is that Apple doesn't offer an export option at all, not in iTunes and not in the Photos app. Your messages only exist inside the phone's backup, so on Windows you have to go at it through the backup rather than through the phone.

What actually works:

1. Plug the iPhone in and open the Apple Devices app (or iTunes if that's what you've got).
2. Tick "Encrypt local backup" and set a password. This step matters more than it looks. Attachments are only included in encrypted backups, so if you skip it you'll get all your text and none of your photos or screenshots.
3. Back up to this computer, not iCloud.
4. The backup lands in %APPDATA%\Apple\MobileSync\Backup\ inside a folder with a long hexadecimal name.

From there you need something that can read the backup and lay it out properly. I built a site for this called messagekeepsake.com. You point it at that backup folder, and it keeps the dates, contact names, conversation order and the photos, then gives you a print-ready PDF. Full disclosure, it's mine, and it's $5 for the finished file. Nothing gets uploaded anywhere, the whole thing runs in your browser on your own machine, and it builds the PDF before it asks for payment so you can see the page count and check it looks right first.

If you'd rather not pay for it, there are free command line tools that can read a Messages database, but on Windows you'd have to extract that database out of the backup yourself first, which is a decent amount of fiddling.