Okular is the new default all-in-one document viewer introduced with KDE4, and it includes the format support that was previously provided by a number of different applications. It replaces KPdf and adds functionality earlier a part of KDVI, KFax, etc.
Okular has support for a large number of document formats, such as PDF, PostScript, ODT, TIFF, DjVu, DVI, ePub, etc. It can even open Microsofts XPS document files, and .chm help files, and can display the popular CBZ comic book files. It is also capable of handling DRM’d PDF files, and accessing files embedded in PDFs. While it is not very suited for the purpose, it can also open most kinds of image files and serve as an image viewer.
Okular claims to be more than just a document reader, and that it is. It supports many annotation options for reviewing documents. Using the review tools, you can perform many of the action that you usually need to do while studying / reviewing documents, you can add notes, add inline notes, perform free-form highlighting, perform text highlighting or underlining, overlay lines and polygons, add stamps, or encircle parts of documents.
For a windows use, Okular can serve as a good replacement for Acrobat Reader or Foxit, and can serve as a handler for any .odt documents you have if all you need to do is view them.
