In the U.S. at least (obviously not the same everywhere), fair use doesn’t necessarily require your work to be transformative. It’s one of several aspects that gets considered, albeit a fairly significant one in many cases. Downloading books/research articles/pirated works in general wouldn’t be fair use as the purpose of the act (obtaining a book to read) directly impacts the market for the work (selling books). There could still exceptions in some cases, mostly related to teaching I’d imagine.