Okular is a KDE universal document viewer based on the code of the KPDF application. Although being based on KPDF code, Okular has some unique features such as overview mode, improved presentation support and annotation support.
Okular supports a lot of different formats like PDF, PostScript®, Tiff, CHM, DjVU, Images (png, jpg, etc.) XPS, Fiction Books, Comic Book, Plucker, EPub, Fax and Markdown. For all supported formats and their features see Okular Document Format Handlers.