How to display text file content saved on account?

Exactly how this works will depend on what library/SDK you're using, if any.

For example, if you're using the official PHP Core SDK, you would use the getFile method to access the file content. That method writes the data out to a resource that you provide, and what you do with that is up to you. E.g., you could save it to a local file, or directly use or display the data elsewhere.

Or, if you're using one of the HTTP endpoints directly, e.g., the /files/download endpoint for API v2, the data is returned directly in the response body, at which point you can likewise do with it whatever you'd like.