Pass interference
 

Can a db level a receiver before the pass is thrown? Would knocking down an eligible receiver be the same as holding?

If you are talking high school rules, then the answer is yes under the right circumstance. If the receiver is a potential blocker, the defender can engage that receiver by knocking them down. If they are running a route (looking for the ball, not trying to block) then they cannot be hit anywhere on the field.

The penalty if done illegally it would be "Illegal use of hands" which is the "Holding" signal.

Peace