Hex editor is a special type of editor that can open any type of file and display its contents, byte by byte. You can see these invisible characters (and regular characters, too) with a hex editor, where they appear as hexadecimal values.
You can use Exploit Pack's hex editor to quickly view and edit binary files. Go to File -> Open and select the desired file to be used by the editor:
After that the file will be loaded into the Editor as shown below: