Select the Characters -> \uFFFF command in the Convert menu to replace all non-ASCII characters in the file with an escape sequence in the form of \uFFFF. If you select some text before using this command, only non-ASCII characters in the selection are replaced. Otherwise, all non-ASCII characters in the file are replaced.
Using this command does not change the encoding EditPad Pro uses for this file. Saving the file after replacing all non-ASCII characters this way only results in an ASCII file if the encoding you're using for the file is compatible with ASCII. For example, the Windows code pages will result in an ASCII file, but UTF-16 will not.
In Convert|Text Encoding you can select "ASCII + \uFFFF Unicode Escapes" as the file's encoding. When using this encoding, EditPad Pro displays the actual characters for all \uFFFF escapes in the file. When you type non-ASCII characters into the file, EditPad Pro will display the character you typed, but store its \uFFFF escape in the file. If you type "après-ski" using this encoding, EditPad Pro will display "après-ski", but store "apr\u00E8s-ski" in the file. If you open that file in Notepad (which cannot interpret Unicode escapes), you'll see "apr\u00E8s-ski".
Use the Convert|Characters -> \uFFFF along with an encoding such as UTF-8 or Windows 1252 when you want to use Unicode escapes in only part of the file. EditPad will display the Unicode escapes while you edit the file. You can see where Unicode escapes are used and where actual characters are used.
Use the "ASCII + \uFFFF Unicode Escapes" encoding when working with files where Unicode escapes are the only way of representing non-ASCII characters. EditPad will display the actual characters while you edit the file. You won't see the Unicode escapes. EditPad makes sure all newly typed or pasted characters are stored as Unicode escapes in the file.
If you have a file that contains non-ASCII characters that are not written as Unicode escapes then you can convert that file you being pure ASCII + \uFFFF Unicode Escapes in three steps. First, open the file in EditPad and make sure all characters are displayed correctly. If not, use Convert|Text Encoding to select the proper encoding with the "interpret" option. Second, use Convert|Characters -> \uFFFF to convert all non-ASCII characters to Unicode escapes. The actual escapes will appear. Finally, use Convert|Text Encoding and select to convert to ASCII + \uFFFF Unicode Escapes. The Unicode escapes now again appear as actual characters. If you save the file the, it will be saved as pure ASCII with Unicode escapes.
In the Encoding section in the file type configuration, there is an option to make EditPad Pro detect whether a file consists of pure ASCII with Unicode Escapes. If that option is on, EditPad Pro automatically selects the "ASCII + \uFFFF Unicode Escapes" encoding for such files and displays the actual characters rather than the escapes. By default, this option is off for all file types except the Java file type. In Java source code files, Unicode escapes are 100% equivalent to actual characters, even for ASCII characters.
Convert|\uFFFF -> Characters