Where are the Aircraft files and manuals in MSFS?

The suggested path for this content from the simulator is: 

C:\Users\COMPUTERNAME\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\Packages\Official\OneStore\

This is if you didn't change the path for the simulator content while installing, if you defined other drive or folder for the simulator content then the route will be, for example:

D:\MSFS2020Content\Official\OneStore\

Inside those folders you'll find the Carenado aircraft files, including the manuals.