WD Officepad is described as 'may not be one of the normal text editing applications that you may have likely come across while looking for one. This is because of its unique design principles and build style' and is an app in the Development category. There are more than 25 alternatives to WD Officepad for a variety of platforms, including Windows, Android, Online / Web-based, iPhone and Mac. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Evernote. Other great apps like WD Officepad are CherryTree (Free, Open Source), Cryptee (Freemium, Open Source), Notejoy (Freemium) and MemPad (Free).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
NixNote, formerly Nevernote, is an open source client for Evernote . It is written in Java so it will also run on Windows & OS-X, but the primary focus has been to try and get a usable environment for Linux.