What is it?Here what there website says:
Okular is a universal document viewer based on KPDF for KDE 4. This means Okular works on multiple platforms, including but not limited to Linux, Windows, Mac OS X, *BSD, etc.Okular gives you various options such as annotation, text highlighting, text extraction etc.
Okular combines the excellent functionalities of KPDF with the versatility of supporting different kind of documents, like PDF, Postscript, DjVu, CHM, XPS, ePub and others.
The document format handlers page has a chart describing in more detail the supported formats and the features supported in each of them.
To install Okular in your Ubuntu:
sudo apt-get update sudo apt-get install okular