Okular is the document viewer for KDE SC 4 (although it does not require the full KDE desktop). It is based on KPDF and it replaced KPDF, KGhostView, KFax, KFaxview and KDVI in KDE 4. Its functionality can be easily embedded in other applications.
Okular was identified as a success story of the 2007 Season of Usability. In this season the Okular toolbar mockup was created based on an analysis of other popular document viewers and a usage survey.
Okular's annotation features include commenting on PDF documents, highlighting and drawing lines, geometric shapes, adding textboxes, and stamps. Annotations are stored separately from the unmodified PDF file, or (since version 0.15 with Poppler 0.20) can be saved in the document as standard PDF annotations.
Text can be extracted to a text file. It is possible to select parts of the document and copy the text or image to the clipboard. Other features include trimming of white page borders and setting of bookmarks.
The official version obeys the DRM restrictions of PDF files by default, which may prevent copying, printing, or converting some PDF files. This can be turned off in the options under "Obey DRM limitations", however.