They're in game/images/Misc/niki[1-5], I'll transcribe them in a couple of hours. The folder is full of images that are flashed on screen quickly or are overlaid on top of images, it shouldn't be too spoilery unless you go out of your way to look at the full size images.
But you get to see the letters without having to do any kind of shenanigans after you experience one update worth of events at the second christmas event (halloween 2 -> november (you are here) -> christmas 2). feels kinda fitting that if you want to read the letters before the game gives it to you that you should do a little work for it.
Optional non canon content that is fun to read through the scripts of if you've reached chapter 4 and none of it counts as game spoilers. Some of it is neat since it gives you entirely new ways of viewing characters, or more of stuff that only appeared for a moment (I'm a fan of both Maya scenes and reading how confident she is outside of the resets)
If you load them into the game instead of just reading the scripts of them your saves will be permanently deleted, if you asked the question instead of figuring it out yourself you probably shouldn't trust yourself to disable the drm.