So long as you're actually using HTML (and not literal plain text) all you need to do is add a viewport meta tag:

    <meta name="viewport" content="width=device-width">

Mobile browsers set the viewport width to a default of 960px (Android might be a little different from iOS). That's why a plain text file displays so poorly on mobile. It's rendering to a pixel width larger than the display (and zoomed to fit).

For the "plain text" look set the font family to monospace. Bam readable and responsive "plain text" look.