okular is an universal document viewer based on KPDF for KDE 4.
Its development began as part of Google's Summer of Code program. The description of the project is located at KDE Developer's Corner.
okular combines the excellent functionalities of KPDF with the versatility of supporting different kind of documents, like PDF, Postscript, DjVu, CHM, 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.
