Select the Characters ->  command in the Convert menu to replace all non-ASCII characters in the file with a decimal character reference in the form of . 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 +  NCR" as the file's encoding. When using this encoding, EditPad Pro displays the actual characters for all  character references in the file. When you type non-ASCII characters into the file, EditPad Pro will display the character you typed, but store its  NCR in the file. If you type "après-ski" using this encoding, EditPad Pro will display "après-ski", but store "après-ski" in the file. If you open that file in Notepad (which cannot interpret numeric character references), you'll see "après-ski".
Use the Convert|Characters ->  along with an encoding such as UTF-8 or Windows 1252 when you want to use numeric character references in only part of the file. EditPad will display the numeric character references while you edit the file. You can see where numeric character references are used and where actual characters are used.
Use the "ASCII +  NCR" encoding when working with files where numeric character references 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 numeric character references. EditPad makes sure all newly typed or pasted characters are stored as numeric character references in the file.
If you have a file that contains non-ASCII characters that are not written as numeric character references then you can convert that file you being pure ASCII +  NCR 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 ->  to convert all non-ASCII characters to numeric character references. The actual escapes will appear. Finally, use Convert|Text Encoding and select to convert to ASCII +  NCR. The numeric character references now again appear as actual characters. If you save the file the, it will be saved as pure ASCII with numeric character references.
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 numeric character references. If that option is on, EditPad Pro automatically selects the "ASCII +  NCR" encoding for such files and displays the actual characters rather than the numeric character references.
Convert| and  -> Characters
Convert|Characters -> &htmlchar;
Convert|&htmlchar; -> Characters
Convert|Reserved Characters -> XML Entities
Convert|XML Entities -> Reserved Characters