The use case for 99% of pdfs is email transfer. They are absolutely superior to sending a clunky, bloated MS Word or CAD document. The web archive is just the final resting place in the process that made them.
With academic articles, I virtually never want to simply read them online.
I need to save them for future reference, read them later when I've set aside time, annotate them, refer back to my annotations four months later...
Arxiv (or JSTOR or wherever else) is just where you get the papers. It's not where most academics are going to be consuming them.
(For consumption, a full-size tablet like an iPad, with a stylus or Apple Pencil, is absolutely ideal.)