Salamandrine 

Mechanic here. I won't argue engineering.

They are pretty difficult to straighten. Best I've could ever do was straighter than they were before. It's generally not worth the trouble.

Anyhow, don't try to do it on the crank arms unless it's a really cheap bike you don't care about. You are more likely to bend the crank arm then the pedal shaft. *

Best case: disassemble the pedals, screw the axle into a big chunk of steel that is tapped to take the pedal. Put that into a huge vise, and straighten the axle with a heavy pipe. (of course most people would use an old steel crank arm instead of a custom chunk of steel)

*BTW, sometimes it's the crankarm that is bent, not the pedal.