Datasheet websites do that a lot. If it's PDF.js, Firefox's PDF viewer (or a fork of it), I just right-click to "Show only this frame" and it goes fullscreen. It might have shenanigans such as disabled printing but you can press Ctrl+Shift+E and reload to check network activity for what address the PDF is loaded from and save that.
The worse ones are PDFs that exist only for SEO and contain nothing but keywords and a link to a paywall.