Locate the file location using Terminal app. Use the following built-in openssl command to encrypt the file;
openssl aes-256-cbc -salt -pbkdf2 -in name -out name.aes
You will asked for password for twice. Do not forget the password otherwise you may loose access to your file after all.
To decrypt the file use the following command;
openssl aes-256-cbc -d -salt -pbkdf2 -in name.aes -out name