I feel the need for the sci-fi campaign next, but I don't want a 'build your own galaxy' ruleset.
I would like a defined setting, and rules that lead a lot closer to reality than fantasy. Definitely no magic.
Looking for a Firefly (TV series) feel.
Coriolis, Alien RPG, GURPS, and Mothership RPG are out, already looked at them.
Knowing you restrict yourself to PDF only...
Traveller is the closest feel (as in, what you're looking for is probably better described as a
Traveller feel), and the hardest-science version of Traveller is Traveller: The New Era... it's still got fusion rockets and gravitics, and there are psionics rules, but said psionics rules are easily ignored, and only a minor part of the official setting. On the other hand, the ability of the deo-chips to transmit a virus that rewrites the chips of the receiving computer/comm-gear... pretty much requires a (non-present) psionics component to be sensible.
Which said, setting wise, there's a HUGE raft of it for Traveller... but most of it isn't regressed to TNE levels... it's the post-holocaust version of Traveller.
Albedo (the Chessex/Thoughts & Images editions) are fairly hard SF; the Albedo Ship Sourcebook is the hardest SF ships supplement I've seen. And yet, it does so with minimal crunch. There's over 500 pages of the comic book online at the author's website:
Albedo . There is excellent setting in the Chessex/T&I version (Chessex is the 2nd ed, T&I the first). But it's an anthro setting (with a science based reason - humans existed in the setting). The system mechanics are not as smooth as I'd like, but it's quite playable, and is set between wars between the EDF and the Independent Lapine Republic.
Note also: GURPS is about the hardest mainstream SF you're going to find that isn't attaining hard-sf by being rules light... and thus leaving setting restriction to the GM saying "No." No version of Traveller is as hard SF as GURPS Terradyne or Transhuman Space.