All Office365 emails (and thus, emails sent from either your outlook web client or outlook desktop client) are encrypted by default with TLS. This is transport layer security, which encrypts an email from the point at which you send it until it is received by the correct individual. If you want redundant encryption, you can encrypt a file themselves before attaching them to the email – this would require you to also send a password to the recipient, so they can unencrypt the files after receiving the email. It is suggested to send two separate emails if you choose to do this. The first email telling them what you’re going to send, and giving them the password to open the file. The second email would contain the file. Encrypting a file would best be done with 7zip and adding a password to it.