A free program for Android, by SkyTek Apps.

The app can read and write to PDF files. It can convert to and from many different formats. This includes to and from text files, as well as images and other files. It is free to use, you can download it from Google Play.

There is a built-in dictionary for most common words, as well as thesaurus and grammar. There is also the ability to use your own dictionary and mark words you want to learn. The app includes many different settings and is highly customizable. It can be downloaded as a trial version and then upgraded. There is a one-time fee to unlock the full version.