Answering because I came to this from Google, even though it was asked ages ago. Check out constellate.org. Recent release by JSTORLabs, they integrate multiple databases, and allow access to filtered sets of up to 25k papers at a time. As far as I can tell you can filter and download sets of this size, so if you want 1M you'd have to do this 40 times. If you want everything, you'll be there forever, but you typically don't need everything.
Alec McGail
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