At first blush it's "theft of services" (among other things), as already pointed out by @MichaelDorfman's answer to Is unauthorized downloading of music stealing? Below's my maybe (or maybe not) more nuanced situation.
I've got lots of perfectly legally purchased Springer-Verlag (among other publishers, but Springer's the worst -- actually North-Holland's the worst, but I have just a few books from them) textbooks, receipts and all. I'd guesstimate their prices (averaged over all publishers, but pretty much the same for each one) have a mean of just under $100USD with a standard deviation of maybe $30.
Springer books are frequently printed on acid-rich paper that turns brown and starts crumbling after 10 or 20 years (ditto several-to-most other publishers). And the bindings similarly crack and break after typical textbook use (ditto again). And since I make copious marginal notes, the books are essentially irreplaceable, making Springer's (et al) shoddy publishing just that much more frustrating.
And you'd think that for the considerable prices they charge, that they could spend the extra ~50cents to manufacture quality acid-free, good-binding books. Personally, I think it's kind of outrageous that they don't. Moreover, I have no alternative when I want a particular textbook -- the publisher's pretty much a monopoly. A completely unregulated monopoly which (like most completely unregulated monopolies) chooses to screw over its locked-in customers.
Okay, so you already know where I'm going with this, now that I have the chance to screw over Springer...
(1) If I already own a perfectly legally purchased, but turning brown and to dust, textbook, then what's the ethics of availing myself of a freely-downloadable copy of that same book? That is, how mitigating are these circumstances with respect to those actions?
(2) Moreover, as an even more general ethical question, does my pretty high level of over-the-years outrage at Springer contribute additionally to that mitigation (i.e., would someone less outraged be less mitigated)? And generally speaking, does more outrage at an egregious situation ethically justify more "affirmative action"? You can receive additional civil compensation for emotional trauma, right? And I'm just acting as my own judge and jury here (and you know how that turns out:)
(3) So, for "emotional trauma compensation", how much less unethical (how much more mitigating) is it to download Springer books that I don't already legally own?
Postscript: After deservedly bashing Springer, I should say that some of their books are indeed well-bound on acid-free paper, e.g., my 1985 hardcopy edition (purchased in 1986) of https://www.springer.com/us/book/9781461577089 is in good-as-new condition after considerable use. Ditto several others, but I'd say ~85% of my Springer books are poorly manufactured, as described above. Cambridge University and MIT Press are maybe 50-50, Oxford UP maybe 70good-30bad, Academic Press 100good, North-Holland 100bad.
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Re comments from @cell and @Bread : it occurs to me that I should have mentioned that I have no intention of actually downloading any copyright-violating material. Else I'd definitely have posted a law.se question about any possible criminal/civil exposure for having such stuff on my hard drives. And even if there were no legal exposure, my computers are part of my home office contract programming business, and I'd never mix up client project files with such questionable stuff.
By the way, some very excellent textbooks are nowadays published under the "creative commons" (cc) license, whereby you can legally download (and even redistribute) copies freely. https://www.researchgate.net/publication/317171939_Foundations_of_Quantum_Theory is one particularly fine example. If you download a copy, you'll see that it explicitly says all that on its copyright page. So the world is definitely beginning to slowly turn in this direction.