This dates back to the old file transfer mechanisms in UNIX FTP. Generally each character in an ASCII file is restricted to using the lower 7 bits of each byte. All of the standard English characters can be represented using ASCII. When one begins using characters requiring more definition than 7 bits per byte, the file becomes known as a binary file. It could be fonts, unicode characters or control codes in the file that exceed the 7-bit limit. Try looking at the file in Wordpad and then in Notepad ... Notepad would automagically ignore non-ASCII characters or present them as little boxes intermangled with the text.

If you saved the file from Notepad, you should have an ASCII file as a result. (I think...?)