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Not exactly a primary source but according to this thread: https://forum.nasaspaceflight.com/index.php?topic=10617.0

"Toner is held to the paper in a laser printer by electrostatic attraction (opposite charges attract), not gravity. Along those same lines, in an inkjet printer ink droplets are fired at the paper, not just dropped, so once again I doubt gravity is an issue."

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But before you even got to the point of printing, the paper in a regular laser or inkjet printer wouldn't even feed properly if upside down!

I'm also willing to bet that whether or not the droplets are "fired" at the paper or not, the inkjet cartridges aren't going to work in an inverted position: they might not even supply ink in that configuration.

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