PDF, DOC, and PUB are the most popular file formats for e-books. But there are other types of e-books available as well. If you have dealt with scanned e-books in the past, you are probably familiar with the DJVU file format. Here are 3 apps you can use to open these files on Android:
EBookDroid: an open source application that lets you view XPS, DjVu, PDF, and comic books. It looks decent and does what it promises.
VuViewer: a simple document viewer for Android. It can handle DjVu and PDF files.
Orion Viewer: a pretty decent PDF reader that can handle DjVu files as well. It works in portrait and landscape modes. You get a built-in file manager too.
DjVu files may not be as common as PDF files. But if you need to open them up on your Android device, the above apps can help.