They seem to be specifically talking about the case where you're on a web page, you click a link to go to what ought to be another web page, but instead you're

They seem to be specifically talking about the case where you're on a web page, you click a link to go to what ought to be another web page, but instead you're in a PDF in your browser. I get it, PDFs are documents and not web pages, and dumping a visitor into a longish PDF when a concise web page with the answers they're looking for would be better. So, use web pages for presenting information best presented in a web page, and use PDFs for presenting information best presented in a PDF, and don't use a PDF when a web page would be better and don't use a web page when a PDF would be better. But that doesn't seem to be the point they're making for some reason.