^^
Peach isnt a character you specifically combo. For the most part, you want to keep her above you because she is floaty, and her d.air isnt the greatest thing ever. One thing you have to realize about juggling a peach is that you have to actually read when she's doing to d.air (or do anything to get out of the air), and to keep her up in the air doesnt mean you have to SH u.air. You might have be able to slide in FH u.air, u.smash, u.tilt, dash attack, etc etc. You goal is to just keep her in the air.
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As for once you get in, that's kinda a funny thing. You dont specifically WANT to be in on peach. Peach has plenty of means from stopping you from getting in, and she has plenty of means of ****ing your *** up once you get in. However, once you get away from her, she cant really do all too much to keep hitting you so long as you dont go towards her. Now, that's not saying you must flat out run from peach the entire matchup, but it's another way to look at the matchup.
With that being said, you dont ever really want to be "in" on peach. Realistically, all you want to do is f.air.
F.air
F.air
F.air
F.air
F.air.
Why? Because in the event she wants to CC, she's going to get pushed out unless she's at low %s (cause she's going to block it or get pawnched in the face). So if you want to get "in" you F.air. Period.
Now, that's not saying dont use other aerials, but you have to use them with the intent of not landing next to her. I.E., FH D.air if you read her being >>>> _____ here in the air, b.air to stop her from floating in, etc etc. But you really should not be allowed to do something like n.air and land next to her (Hell, you shouldnt be allowed to do that to anybody, but peach is high on that list).
As for pills, because of how slow peach is, she has to be fairly close to punish you for pills. The only exception to this rule is if she pulls toad. And ironically, toad has more range than anything Doc has. So feel free to pill, but if she's not in shield, keep the "ace in the hole" in the back of your mind.