ArXiV files are downloaded in either .PDF or maybe .HTML. In my subfield (computer vision), most of the action happens in actual journals/conferences for now, so when authors submit to ArXiV, they submit .PDFs because the work usually was also intended to appear somewhere else.
ArXiV does not specify a style guide so you get this weird mix of IEEE/PAMI/single-column/double-column, but that's not really a detrement to its readability since journals wouldn't usually pick an unreadable style anyway.
Thanks, though I'll note you weren't who I was asking the question of ;-)
I figured the format was likely "academic articles, mostly prepared with LaTeX, published as PDFs", but the commenter was being less than clear, even on reiteration.
