PDFs are great for typesetting for print, where you know the paper size and adjust everything pixel perfect to it. Nothing beats PDF when it comes to complex typesetting for print. Web pages are meant to reflow and much better for reading on smaller screens. Also modern web technologies can go far beyond a PDF when it comes to interactive/dynamic content, but web pages (sites) are also cumbersome for a non-technical user to download for offline use with all elements intact.
But I suspect HTML will eventually win this. While HTML can be printed, PDFs will always struggle with changing device sizes. Plus the web is becoming more of an app as time passes while PDFs will probably remain dumb content due to security reasons, so their applicable niche is growing smaller as the Web creeps in scope.
It's not true that PDFs are only for print. In fact, MacOS display technologies are based (at least the first iterations) on PDF. PDF for screen can work very well, the problem is that the industry never standardized this aspect of the technology. The result is that viewing PDF nowadays is far from optimal. I truly believe that PDF could have been a much better technology than HTML for modern websites. Instead HTML, which started as a semantic technology, was shoehorned into what we have today.
But I suspect HTML will eventually win this. While HTML can be printed, PDFs will always struggle with changing device sizes. Plus the web is becoming more of an app as time passes while PDFs will probably remain dumb content due to security reasons, so their applicable niche is growing smaller as the Web creeps in scope.