Include file in posts and pages

Hey everyone!

So I’m looking all over for a solution and everything is outdated or doesn’t work.

We have boiler plate ‘about us’ text we want to include on all our posts. Obviously, if there’s a change to this, I don’t want to make a change to ALL posts so I’m looking into some way to just include a single file under each of the posts. I’ve tried php include ‘xxx.html’ and tried a plugin with a 5-star rating to no avail.

Does anyone have a solution that you’re currently using and you’re sure is compatible with latest WP update?

Thanks!