That's a good question, I'm not sure. I just make sure the file has the correct extension and it's opened with the correct app. I've never had an issue on android but I mostly download from the same site which seems to have good files (and always with adblock).
MicrowavedTea
Specifically for books I pirate because I get a better product this way. I prefer reading on my phone and downloading an epub means I can open it in any app I want, add chapters and share it with whoever I want. If I could easily pay for a book and get the same experience without any drm or online account bullshit I would probably do it.
Physical books are also ok but buying anything not in my language means possibly waiting forever for it to arrive and paying more for transit costs. I may still do it if I really want to support the author but I'd rather have a way to pay them directly tbh.
You can import a certificate authority through settings -> security -> view certificates. I don't remember if this works with self-signed certificates though. You can also manually add exceptions there.
Btw there is a nice guide for the reading order and connections of the main Discworld books. While I haven't read Night Watch, Guards! Guards! is also very good and it sets up the watch story nicely so you could start from there and maybe skip a few books before the Night Watch.
It doesn't have to be a big emergency, FM can still be very useful. A few years ago there was an earthquake in my city that wasn't too big but we still had to evacuate the building. I don't remember any damage in my area but because everyone was trying to make calls there was ZERO cell service. So a bunch of people were gathered around my phone listening to a news station for updates because I was the only one who had brought headphones. Without FM this ability is now lost.