This app was mentioned in 2 comments, with an average of 3.50 upvotes
Here's a text to PDF converter for an iPhone. Sketchy looking website, but I downloaded it awhile back with no issues. If I recall correctly, there's some part of it requires a purchase, so the "free download" thing is kind of misleading.
I also found this app if you have an Android phone. I don't have any experience with this app, it just came up in my search results.
Hopefully these help!
You could use something like this to create a document with your texts in it. Then from there any word stat solution would be viable.
https://play.google.com/store/apps/details?id=com.smsconverter.lite