As you start off with “I have to create a file to send” I would say the encoding of the file is entirely up to you. You could use ASCII, EBCDIC, Unicode, etc. If you’re creating the file on an i then EBCDIC will generally be the easiest for you (EBCDIC is the default if you’re creating the file using a high level language such as RPG)..
One thing to keep in mind is that ASCII, EBCDIC, and Unicode are really rather high level indicators of how the file is encoded. If the “location outside of our company” is in the same country (or really, using the same national language) as your company then saying it’s ASCII or EBCDIC might be sufficient. If your company is in the United States and the other company in Japan, Russia, England, or a long list of other countries then they may need to know the CCSID of the data you will be sending them (which you can control) depending on what type of data you’re sending. With Unicode, even within the same country/national language, they may want to know if the data is UTF8, UCS2, UTF16, UTF32, etc (though they can typically figure this out with one quick look at the data received).