Okular

Okular is a universal document viewer developed by KDE as part of the KDE Applications suite. It is cross-platform, primarily used on Linux and other Unix-like systems, with ports to Windows and macOS. Okular is built on the Qt framework and utilizes KDE Frameworks; for rendering it relies on Poppler for PDF and PostScript formats and on other libraries for additional formats.

Okular supports a broad range of formats, including PDF, PostScript, DjVu, CHM, XPS, ePub, FictionBook (FB2) and CBZ/CBR for comic books, among others. The viewer provides unified navigation, including table of contents, thumbnails, and search. It supports continuous and single-page viewing, and allows multiple document tabs or windows.

Annotation and markup: Okular provides annotation tools such as highlights, underlines, strikeouts, freehand drawings, shapes, stamps, and inline notes. Annotations can be stored within the document or in sidecar files and can be exported or imported for sharing. Text selection, copying, and a built-in accessibility-friendly interface are supported.

Printing and related features: Documents can be printed, and content can be exported to images or other formats in some cases; text extraction and copying are supported. Okular also supports form data display in certain formats.

License and development: Okular is free and open-source software released under the GNU General Public License. It is actively developed by the KDE community with a focus on usability, performance, and integration with the KDE desktop environment.