If they jump into the stage > up-air, dair, nair or even a thunder. You need to know if they're going to throw out a hitbox or use a defensive option and then chose the best punish option
If they jump in front of the stage (characters like Pikachu or Diddy that have a special with great mobility might pick this option to get back to the stage) > out-stage fair, or wait to see their action and then punish (since they can't retreat to the ledge, they're probably doing something laggy, in which case you can punish with Pikachu great mobility).
If they do a regular get up > f-smash, f-tilt, dash grab, dair. Normally they do a regular get up when there's some space between us and the ledge, which makes them feel safe to just do a regular get up. In this case, I normally try to see their reaction after the get up and then punish. At first, people will usually shield (punish if grab), spot dodge (punish with many pika-stuff, like up-smash, f-smash, grab or dair) or roll (punish with dash grab, up-smash, f-smash) after the regular get up, and after they notice they're getting punished they'll try to incrise their mix ups by throwing ou hit-boxs (just wait and then punish, or use a f-smash) or jumping (start a aerial string, or run and do an up-smash as they jump).
If they roll > dair, f-smash, grab, up-smash. They usually roll when we are right above the ledge, specially if we're throwing out hitboxs (like the rapid jab or tilts). Since Pikachu has a small ending lag on his tilts/jab, we have more than enough time to run and punish the roll.
Overall, Pikachu does not have a move that covers every single option of the oponent (dair is probably our best option if we're just guessing). However, he has the mobility and fast moves to quickly switch between ledge-cover options, and he has great punishes at all %, since he can combo out of a grab, aerial, or tilt, and can kill with a smash attack.
Just try to condition your opponent into doing what you want, and then pick the best punish you can think off.