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Even if you managed to pump Mars's atmosphere up to the thousand-millibar pressure of the Earth's, you couldn't breathe the stuff. It simply doesn't contain what people need for breathing.
The first thing you think of wanting is oxygen, of course. Well, actually, there's oxygen in Mars's air. In fact, more than half of what atmosphere there is on Mars is oxygen—kind of. The trouble is, that oxygen is already tightly locked up into carbon dioxide. Only 5 percent of the negligible quantity of air Mars possesses is anything but that useless carbon dioxide, in fact, and a lot of that remaining 5 percent is equally useless stuff like argon. No, that won't do to support human life. What Mars needs to make it green is hydrogen, to react with some of that oxygen to create water, and nitrogen, to mix with the rest of the oxygen in order to reduce the air to something people could breathe without burning out their lungs—and, oh, yes, so that you can eat, because plants won't grow without nitrogen.