As for notepad, the solution is to make the file bigger than 64K 🙂
Generally, normal text editors (those you menthioned, too) can read any file. But in case it contains some binary data instead of words & letters the user will see a lot of ASCII symbols, making no sense for him/her.
If you've got some textual data you want to make unreadable, you have to encrypt it. Depending on what level of security, it may be something really simple (for example, replace every letter with one next to it in alphabet), or some really complicated encryption algorithm used in real world applications. Honestly speaking, every decodable encryption algorithm can be hacked, but hacking may require too much computer power, thus being unreasonable.
Anyway, it is cryptography matters, and not a question for PHP forum; besides, I haven't got much knowledge on the topic. And anyway, I believe you don't want to go this deep in the matter...