Whilst most applications (like Word, Excel and Adobe) allow passwords to be set from within the application to encrypt them, some files like flat text files (CSV etc.) do not support this process, so you need to add the file to a compressed (ZIP) file and use this to add a password to encrypt it
This process also applies it you have multiple files/folders you need to encrypt into a single file - just start from the folder containing all the files/folders
Locate the file/folder on your computer (desktop, documents or USB drive - not O365 shared area) and right click on it to bring up the context menu, then select '7-zip' and 'Add to archive...'
Then in the box that appears set the filename you want at 1, then the archive format to 'zip' at 2, then enter the password you want (twice) at 3, set the encryption method to 'SHA-256' at 4 and then click on 'OK' at 5
The zip file is created in the same location and can now be used to send to your intended recipient
Whilst the zip file can be opened - to see the content file(s), the password column confirms it is encrypted
And nothing can be extracted without entering the password when prompted