I want to add descriptive text (non-visible, for SEO purposes only) to several individual pages I've created for a site - so I have individual pages such as info.php, about.php, publications.php, etc.

Each page will get its own custom text. (I realize that Google doesn't think much of meta tags anymore, so I'm not planning to insert custom meta tags...)

What's the best way to do this? None of these custom pages contains any images.

Google does not think anything good about invisible text for seo purpose either. This is really an ancient measure outdated for years...and hurts more than it helps you. Use proper content someone finds useful or similar is the best you can do, even if the text is styled after seo purposes. The site afterall is for people and not machines, isn't it? Google writes this in their own guidelines btw.

Regarding how to do, I actually don't understand what you are asking. You say about.php and similar so these are static custom theme pages. Then add that content as you add any other content, too.