Connecting powered off phone to PC
No, while the phone's powered off you can't read the files on it. If your bootloader is already unlocked, you might be able to find a custom recovery that just lets you mount as a USB drive. In that case, booting into the recovery would be sufficient even if the main OS doesn't boot. Of course, if your bootloader isn't unlocked, unlocking it to flash a custom recovery would wipe the storage anyway.