UI elements are still far smaller than comparable ones in Windows 10 or macOS, when those are running at 1080p unscaled. On XP, IE and Explorer both manage to fit four toolbars (rather, a title bar + three toolbars) in 105px of vertical space. On my own macOS Catalina install, Chrome manages to fit only two toolbars in 86 device-independent pixels of vertical space.
Likewise, just consider the fact that people weren't generally looking at PDFs back in the XP era, but rather plaintext, which—as rendered on this image—was exactly 8px tall. A lot of 8px-tall text fits on a 1080p display, no matter how many toolbars you have; and a lot of 8px-tall text will even fit on a 640x480 display, if you make efficient use of the vertical space. But because PDFs use vector fonts, not bitmap fonts, they will look unreadably muddy if you attempt to scale one down such that the text only uses 8px of vertical height. You need more vertical space to look at PDFs.
So, in combination, when you throw a Windows 10, running Google Chrome, displaying a PDF, on a 768p display, it's just an ugly constrained problem.
When done right, increasing screen density doesn't really affect available space, just makes everything sharper (unless you manually reconfigure it to get more space, like I do on my hidpi laptop).