There's nothing about telekinesis or mage hand that says you couldn't push an object towards the ground. However, you wouldn't be getting any benefit for pushing in the same direction of gravity. The spells both have a listed force (5 lbs for mage hand, 25/level for telekinesis, 100/level for t.haul) so that's the magnitude of force where those things would cap. Also, it's magic, the weight of your item is not part of the equation.
As far as pinning people beneath rubble, none of these would be particularly effective:
Mage Hand a) isn't strong enough to do much, and b) can only move "unattended" objects, so if someone is actively trying to shift rubble you can't even target that piece.
As for telekinesis, look at the three ways it can be used:
-a gentle, sustained force
-a variety of combat maneuvers
-or exert a single short, violent thrust
I guess if your GM was reaaallllyyy flexible you could try to do an overrun combat maneuver with someone stuck under rubble but I can't imagine too many people would let that fly.
If you really want to be that cruel to someone who's buried alive you could just pile more stuff on the rubble heap.
Your best bet is to position the object over a target and then release it.
As per the rules on CRB p443 it would require a ranged touch attack and the damage would depend on the size of the object and the distance traveled.