My new application Data Safe available for free download on Firefox Marketplace.
This app allows you to choose files from your phone and encrypt them with a pass phrase.Also you can decrypt the encrypted file.
Note that the app does not encrypt the actual file, but a copy of it, so you would not lose the original. You can encrypt any file from documents,music,videos,pictures,etc.
You can download the app from: https://marketplace.firefox.com/app/data-safe/
The article related to the app was also covered in newspaper “Maharashtra Times”