They can't force you to tell them your PIN in some countries, but they can try all PINs, and they can search your desk drawer to find the post-it where you wrote your PIN.

Good PINs are ones you're not allowed to brute force. You can easily configure an iPhone to wipe itself after too many wrong guesses. There's a single checkbox labeled "Erase Data", saying "Erase all data on this iPhone after 10 failed passcode attempts."

You bet I have that enabled.

They can also hold you in a jail cell until the end of time until you give it up, in many places.