What software can I use to view epub documents?

You can use calibre software for viewing .epub documents.

To install calibre from terminal:

sudo apt-get install calibre

Or click the icon below.

calibre

If you don't want to launch the full calibre client just to view your ebook files you can add a .desktop launcher to calibre's inbuilt ebook viewer:

1. Create a new .desktop file in ~/.local/share/applications:

   gedit ~/.local/share/applications/calibre-ebook-viewer.desktop
   

2. Copy and paste the following passage into the file:

   #!/usr/bin/env xdg-open
   
   [Desktop Entry]
   Version=1.0
   Type=Application
   Terminal=false
   Icon=calibre
   Exec=ebook-viewer
   Name=Calibre Ebook Viewer
   Comment=Display .epub files and other ebook formats
   MimeType=application/x-mobipocket-ebook;application/epub+zip;
   StartupWMClass=calibre-ebook-viewer
   

3. Save the file. You should now be able to launch Calibre's inbuilt ebook viewer both from the dash and through the right click "Open with" menu.

I recommend fbreader. Small, fast, single key page turning. Quite pleasant.

Use the command line to install FBreader:-

sudo apt-get install fbreader

I prefer Okular, a document viewer made for KDE. It has features including bookmarks and highlighting.

It's available in the main ubuntu repository and can be installed on a plain vanilla Ubuntu installation; kubuntu is not required.

To be able open epubs with okular, you must also install the okular-extra-backends package.

    sudo apt-get install okular okular-extra-backends 

Another useful option, try this in Firefox; https://addons.mozilla.org/en-US/firefox/addon/45281/, a fairly good extension for the Firefox web browser that lets you read .epubs from the browser, especially good since on Ubuntu you are likely to be using Firefox and it is usually kept open when you are using your computer.

MuPDF can read epub files, and is only 3 MB.

It may be too minimalistic for some, but I loved it! Way faster than xpdf!

sudo apt-get install mupdf