can I download all my text messages (SMS) from my iPhone or email them?

If your pc is a macOS, you see the Apple articles below:

Use Universal Clipboard to copy and paste between your Apple devices

Use Universal Clipboard to copy and paste between your Apple devices - Apple Support

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac – Apple Support (AU)

From the Mac, you can just copy and paste the text into Pages then print it out, or Save as . . .

Finder -> Apps -> Messages -> select a messages -> copy -> paste to xxx.

If you have multiple messages, you must select, copy & paste per message.

Form iPhone -> You can take a screenshot and print it out.

If your pc is a Windows OS, you need a 3rd party app.

You can take a screenshot and email them.

If you have a mac you can download a thread to a pdf and email