you can't download that, it doesn't exist on the server. The server sends the HTML, the browser's job is to display it. And part of that (can be) is showing the text.
In fact, many web pages are rather empty, and load the relevant content as you read along.
So, what you'll need is a working browser, which displays your text, then you need to get that text.
You'd usually do that by actually remote-controlling a browser from a scripting language: you start the browser in a special "daemon" mode, you connect to it, and using a specially crafted browser control interface (WebDriver) you tell it to go to a URL, wait a second to let the browser render what you'd see on screen, normally, and then tell it to save as a plain text file.