The whole point of a pdf is to display _exactly_ the same across all devices. For something like a scientific research paper, you want all figures, etc., to remain in the same place to avoid formatting oddities.
> That’s an important feature, just like reflow. But does the former make PDF World’s most important file format?
It's not just that PDFs are displayed the same across devices, it's that it's displayed the same over time. I can open a PDF generated in 1995 and it will look identical today as it did then. The same can't be said about HTML or Word documents generated in 1995.