There are badge boxes scattered around RPD where one of three badges may spawn, you'll know its the right box when you see "badge" in the container name, sometimes the containers have bugged out names but the word "badge" should still be in the name. It looks a bit out of place too, to make it more easily spotted. The badges themselves are very tricky to find as the potential spawn boxes are everywhere around RPD and the entire place is filled to the brim with zombies, it's very likely that you may need to make a few trips around the entire complex to find the badges.

Once you have all three badges make your way to the first floor, to the giant statue next to the stairs. There are 3 round slots in the statue, right click and you'll see an option to insert the badges (they must be in your main inventory, not backpack or anything else).

RUN LIKE HELL as a bunch of zombies will drop on top of you, like an absolute absurd amount. You'll get a text hinting that something made a sound nearby, at least I'm like 99% sure that's what it said (I was too terrified at the mountain of zombies falling on top of me to read the text properly, if anyone knows exactly what it says please tell me).

Go to the third floor and find a staircase leading to the fourth floor. It's blocked off by two badge boxes, and a handful of zombies are waiting for you to move one of the boxes so they can get to you. After moving the box and killing the zombies, go up the stairs and find a lion statue in the hallway to the right. Here you have to solve a 3x3 sliding puzzle. Ensure you found a puzzle piece laying around and have it in your main inventory. There's only one, and you need it to finish the puzzle, the spawn may be randomized but I found it in the basement near a bunch of generators.

BEFORE FINISHING THE PUZZLE make sure you're ready for a massive horde. This time they aren't dropped on top of you, but spill out of the secret entrance instead. There's more than a few dozen undead to fight off, but once you do you'll get to your reward for taking on RPD and its puzzles.

A very well stocked armory, including: a RPD gun magazine to learn how to craft a suppressor and a tactical flashlight attachment for your gun, qibaowangshengdadi (a shotgun, idfk why its named that), benelli m4 super90 (another shotgun), and an absurd amount of ammo. You may find duplicates of the guns depending on your game mode, as well as less ammo (playing on survivor I found thousands upon thousands of bullets and 7 or 8 of the shotguns).

the password thing is a book hidden in the basement of the prison

the room is beneath the security room, the stairs leading down is blocked by crates

you then go to the garage with 2 big robots, read the book and type in the password at the garage door

Thanks to ムム for sharing this! Died trying to invade the prison and haven't had a chance to play PZ from a fresh save since then.

1. Visit the gun store at the north-east side of Raccoon City and gear up. An absurd amount of guns spawn there, and in my experience the front door is always left open when you first spawn. It's right next to the mall and very close to some insanely populated areas, so be careful and stay quiet if possible.

2. For RPD, you will never be able to lure out all of the zombies inside the building for easy looting. I spent 2 in-game days shooting off very loud guns, mowing down zombies, honking horns, and there were still roughly 200 zombies inside the building (more after I finished the badge puzzle).

3. Get some gun mods and perfect your aim if you're having a hard time. Specifically, you want silencers. Noise can be useful, but when you're searching certain large buildings it helps to be able to mow down a horde within 10 seconds using good aim without attracting even more zombies. The reward for solving the RPD puzzles gives you a silencer without needing other mods, although I have not tested it yet.

UPDATE
I'm taking a break from the unstable branch until further notice, this guide will not be updated for a while.