Okular
Okular is a free and open-source universal document viewer developed by the KDE community, designed to handle a diverse array of file formats including PDF, PostScript, DjVu, EPUB, comic books (CBR/CBZ), images (such as JPEG, PNG, TIFF, and WebP), Markdown, and others like XPS, ODT, and Mobipocket.[1][2] Built on the Qt and KDE Frameworks libraries, it emphasizes efficient rendering, annotation capabilities, and cross-platform compatibility across Linux, Windows, macOS, BSD, and mobile environments like Android.[3][4] Originating as a Google Summer of Code project in 2005 under the leadership of Piotr Szymański, Okular evolved from the KDE 3-era KPDF viewer, refactoring its codebase to support multiple document backends while incorporating features from other KDE tools like KGhostview and KDVI.[4] This modular architecture, using specialized generators for each format (e.g., Poppler for PDF and libspectre for PostScript), enables advanced functionalities such as text selection, savable annotations (including highlights and notes since KDE 4.9), digital signatures, form handling, table of contents navigation, and export options for text or images.[5][2] A mobile version was introduced in 2013, later ported to the Kirigami framework in 2017 for better adaptability on touch devices.[4] In addition to its technical prowess, Okular stands out for its sustainability focus; in 2022, it became the first software application worldwide to receive the Blue Angel ecolabel from the German Federal Environment Agency, recognizing its resource and energy efficiency, support for extended hardware lifespans through low demands, and promotion of user autonomy via its GPLv2+ license.[6] The project remains actively maintained, with the latest stable release (version 25.08) in August 2025 introducing enhancements like improved PDF form support and minor performance optimizations, available through KDE Gear packages and community distributions.[1][7]Introduction
Overview
Okular is a free and open-source multiplatform document viewer developed by the KDE community, enabling users to view, annotate, and interact with a wide range of document types across operating systems including Linux, Windows, and macOS.[1] Designed as a universal solution, it emphasizes versatility and efficiency in handling diverse file formats such as PDF, e-books in EPub, comics in CBR/CBZ, and images in JPEG, PNG, and TIFF, among others.[1] Originally positioned as a replacement for specialized KDE viewers like KPDF, Okular consolidates functionality into a single, extensible application to streamline document management without the need for multiple dedicated tools.[8] As of November 2025, the latest stable release is version 25.08.3, issued on November 6, 2025, with bugfix updates including improvements to stability and performance.[9][10] Built using C++ with the Qt framework and KDE Frameworks libraries, Okular supports fast rendering through modular backends and promotes extensibility via plugins for additional formats and features.[7] It is widely adopted in KDE-based environments for its seamless integration and reliability in everyday document tasks.[3] In recognition of its sustainable design, which promotes energy efficiency and resource conservation in digital viewing, Okular received the Blue Angel environmental label from the German Federal Environment Agency in February 2022, marking it as the first software to achieve this certification.[11]Licensing and Technical Foundation
Okular is released under the GNU General Public License version 2.0 or later (GPL-2.0-or-later), which permits free modification, distribution, and use while requiring derivative works to adopt the same license, thereby fostering community-driven development and ensuring the software remains open source.[1][12] This licensing model aligns with KDE's commitment to free software principles, allowing global contributors to enhance the codebase without proprietary restrictions.[13] The technical foundation of Okular relies on key open-source libraries for its cross-platform functionality and document rendering. It uses Qt for the graphical user interface and rendering engine, enabling compatibility across Linux, Windows, macOS, and other platforms.[1] KDE Frameworks provide integration with desktop environments, handling components like configuration management and inter-application communication.[1] For PDF support, Okular employs the Poppler library as a modular backend, which processes and renders PDF files efficiently while separating format-specific logic from the core application.[14][15] Development occurs through a collaborative model hosted on KDE's GitLab instance at invent.kde.org/graphics/okular, where a global community of KDE developers submits code reviews, bug fixes, and feature enhancements via merge requests.[7][13] Since December 2020, Okular has adopted a Calendar Versioning (CalVer) scheme, structuring releases by year and month—such as 25.08 for the August 2025 version—to reflect predictable release cycles tied to KDE's broader development cadence.[16] Okular emphasizes sustainability through low-resource usage and open-source practices, minimizing energy consumption during rendering and idle states to extend hardware longevity.[6] This approach contributed to its receipt of the Blue Angel ecolabel in February 2022, the first such certification for software by the German Federal Environment Agency, recognizing its efficient resource management and environmental design.[6][11] For build and extensibility, Okular supports a plugin architecture where format handlers are implemented as separate modules, allowing new backends—such as those for additional document types—to be added without altering the core codebase.[14] This modular design facilitates maintenance and expansion by the community, with optional dependencies like Poppler integrated dynamically during compilation.[14]History
Origins and Early Development
Okular was initiated in 2005 through the Google Summer of Code program, where student Piotr Szymański, mentored by Chris Howells, developed it as a successor to KPDF, the primary PDF viewer in KDE 3.[17][5] The project sought to unify KDE's fragmented document viewing tools by refactoring KPDF's PDF-centric architecture into a modular backend system capable of handling diverse formats, thereby replacing specialized applications like KGhostView for PostScript files, KFax for fax documents, and KDVI for DVI files.[18][5] Early development emphasized extensibility, with initial post-2005 efforts adding support for PostScript and image formats to expand beyond PDF viewing.[5] Okular was first released as part of KDE 4.0 in January 2008, marking its debut as a viable alternative within the KDE graphics suite.[18] By 2007, Okular was recognized as a success story in KDE's Season of Usability initiative, where usability experts analyzed user workflows and redesigned the interface for greater intuitiveness, including streamlined toolbar layouts and improved navigation tools.[19] As KDE transitioned from version 3 to 4 in 2008, Okular was fully integrated into the new framework, introducing features like bookmarking and basic commenting systems to enhance user interaction with documents.[18] This shift was supported by expanding contributions from the broader KDE community, which grew the core development team and accelerated feature implementation.[18] Prior to the Qt5 migration, Okular had evolved to include initial handling of digital rights management (DRM) restrictions, with basic PDF annotations—such as highlights and text notes—added in version 0.18 in 2013, laying the groundwork for deeper integration of additional formats like e-books and comics.[20]Key Milestones and Recent Releases
In December 2016, Okular underwent a significant port to Qt 5 and KDE Frameworks 5 with the release of version 1.0 as part of KDE Applications 16.12, which improved cross-platform compatibility and incorporated modern user interface elements. Platform expansions continued in September 2019 with the availability of Okular on the Microsoft Store, broadening accessibility to Windows users beyond traditional Linux distributions.[21] In December 2020, Okular adopted Calendar Versioning (CalVer) alongside the broader KDE Applications suite in version 20.12, shifting to a predictable YY.MM release format to align with quarterly cycles and enhance user expectations for updates.[22] A notable recognition came in February 2022 when Okular received the Blue Angel certification from the German Federal Environment Agency, marking it as the first software product to earn this ecolabel for promoting resource-efficient and energy-saving digital practices, such as reduced paper consumption through digital viewing.[11] Recent developments include the KDE Gear 25.04 release in April 2025, which introduced support for PGP/GPG-based digital signatures in Okular, facilitating easier verification and signing workflows compared to traditional S/MIME methods, along with options for custom default zoom levels and improved signature visibility.[23] The subsequent KDE Gear 25.08 release in August 2025 focused on stability enhancements and refinements to format rendering, addressing minor bugs in document handling and performance for various file types.[1] Ongoing efforts in later Qt 5 versions have integrated the Qt Speech module into Okular, enabling text-to-speech read-aloud functionality for supported documents, which requires installation of the Qt Speech library and compatible synthesizers like speech-dispatcher.[24] By 2025, development has emphasized Wayland compatibility improvements across KDE applications, including Okular, to enhance rendering performance and input handling on Wayland sessions.[25]Features
Viewing and Navigation Capabilities
Okular supports multiple methods for opening documents, enabling seamless integration with the KDE desktop environment. Users can open files via drag-and-drop using the file manager's context menu by selecting "Open with... → Okular," or through command-line invocation with theokular command followed by the file path, such as okular /path/to/document.pdf.[26] It serves as the default viewer for PDF and PostScript files in KDE's Dolphin file manager, launching automatically upon left-clicking compatible documents.[26]
Navigation tools in Okular facilitate efficient browsing of documents. Zooming options include "Fit to Page" and "Fit to Width" modes via the View menu, along with continuous scrolling for fluid reading; users can also magnify specific areas using the Magnifier mode or keyboard shortcuts like Ctrl++ for zoom in and Ctrl+- for zoom out.[26][1] The sidebar provides thumbnails of pages for quick jumps by clicking, a clickable table of contents for documents with structured outlines, and search functionality accessed via Ctrl+F to locate text across the document.[26][27]
The rendering engine delivers high-fidelity display with configurable anti-aliasing for text and graphics, enabled in Settings → Configure Okular → Performance to ensure smooth visuals.[26] It supports multi-page layouts such as Single Page, Facing Pages, or Overview modes with adjustable columns, and border trimming features like "Trim Margins" or "Trim to Selection" under View → Trim View for a cleaner presentation by removing excess whitespace.[26]
Bookmarks and history enhance user control during sessions. Users can create and manage bookmarks with Ctrl+B via the Bookmarks menu, which are saved per document and accessible in the sidebar's Bookmarks view for instant navigation.[27] Session history allows quick back and forward movements using Alt+Shift+Left/Right or the Go menu, tracking viewed pages within the current document.[27]
Printing and export functions maintain document integrity. Direct printing is available through File → Print (Ctrl+P), with options to specify page ranges and PDF-specific settings for precise output.[26] Export capabilities include saving the document as PDF via File → Save As without alterations, or using the selection tool to capture portions as JPEG or PNG images.[26]