Taking from another angle. How comfortable are you with someone outside your company having access to those files even if they are encrypted?
If you're nervous then rightly so. You don't want to use FTP.
It's not at all safe to use FTP as a transport medium over the internet. FTP should have been phased out years ago. FTP passwords are transmitted in the clear. So an attacker who obtained that password can download your encrypted files directly from the FTP server for themselves and given as much time as they want, could in theory decrypt them.
Also, they could tamper with your files on the ftp server. How will you trust them to be authentic? They might also delete them or upload some rubbish to your FTP server and fill up it's disk with junk. So, can you white-list the server to only those you trust?
So now your security is entirely up to the encryption of the files themselves and whether you can trust them. If you used simple zip encryption, I'd say you could be in trouble. Don't use a simple password based encryption. Consider something like PGP as a better option.
Also, think about the idea of pulling the files from SFTP if the other party has an SFTP server. Or if you can, upgrade your FTP server with SFTP.
Also, encrypt your most important archives/applications if required. Especially sensitive material. Because sometimes it's the people within your organization who can be the biggest threat. e.g. people leaving the company.