But it does, doesn't it? You add the "twocolumn" option and recompile. Unless your LaTeX is too fancy this will tipically give a very good result (at worst, some figures with hardcoded sizing will be awkardly placed).

I cannot do that when I'm reading a paper written by somebody else, and I only have the produced PDF.

That's why arxiv is a god send, because the source is available there, if the author has uploaded it there.

Science needs a culture of open sharing, the same way physics and math has it.

twocolumn is terrible for mobile.

what's needed are narrow margins.

what you are asking for is called a "round-trip" by some printers.. This was requested the week after PDF was invented! It does work, unless it does not.. the company that invented this technology is apparently infested by MBAs and charismatic nobodies, since they announced they are exiting the type "business" ? Our house of cards is showing.

Check zotero. It has that feature