Here's where it can be found as well as how to collect it! You're clearly going to need it for when the aliens take over.
Fortnite Doomsday Prepper Guide Location
There are two doomsday prepper guides on the Fortnite map but you only need to collect one for this quest. One is at Hydro 16, the other is at Steamy Stacks.
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For the sake of this guide we collected the one from Hydro 16.
This is not a named location on the Fortnite map, but if you drop on the marker you should find it easily enough.
Once you land at the location, head inside the main building, you're looking for an office-type room on the ground floor. There is a computer and a filing cabinet inside which you can see through the window.
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On the ground next to the filing cabinet is the doomsday prepper guide, glowing blue, signalling that you can interact with it. Pick it up and that's mission complete! 30,000 XP is all yours and it's on to the last mission in the questline!