A free app for Android, by Earth2Me LLC.

Crypt4All Lite (AES) is a free, lightweight, and highly portable encryption application for your Android phone or tablet. It supports AES-256 encryption, the most secure encryption algorithm, and has an intuitive and user-friendly interface. With the built-in password generator, you can generate strong and unique passwords, and you can easily change your password if you forget it.

If you want to make an encrypted copy of a file or a folder, you can simply choose to do it in the app or copy the file to the app and then use the app to make the encrypted copy.