How many document viewers do you use? I use SumatraPDF (or Edge) for PDFs, Calibre for ePub, Notepad for Text, and also have some portable document editors.
Wouldn't it be nice if you had one program to view them all? That's exactly what Okular does. It's an open source universal document viewer for Windows, Linux and macOS. The program is made by KDE, a name Linux users should be familiar with, among other creations they are the ones behind the popular Kubuntu (Ubuntu + KDE Software) distro.
Let's begin touring the interface. The sidepanel on the left can be used to jump to the Contents, Thumbnails, Reviews and Bookmarks sections. Select one of the options and the list of corresponding items are displayed in the panel to the right of the sidebar.
The Contents option lists each section/chapter in a document, along with the sub-items, page numbers, etc. The Thumbnail mode pane displays a preview of each page in the document, you can scroll through it and click to go to the selected page. The Reviews pane contain the annotations that have been made on the document. If you don't have any, you can add some by hitting the F6 key or from the Tools menu > Review. Bookmarks are custom links that you have added, i.e., if you bookmark a page it will be displayed in the side-panel for future reference. Hit Ctrl + B to bookmark a page.
You can hide the second side-panel by clicking on the same option twice. For e.g., if Contents is the currently selected tab, click on it again to collapse the side-panel.
The large blank space on the right is the Viewer pane, where your documents are opened. Use the mouse wheel or the scroll bar to navigate vertically. You can pan around using the hand tool. Want to go to a specific page? Enter the number in the small box at the bottom and hit enter.
Right-click and draw a box on an image and you'll be able to copy the contents to the clipboard or save it to a file. The same works with text, but there are additional options like copy the text to clipboard, speak the text, search for the text in the document, or search online for the text using Google or Yahoo or Wikipedia or YouTube.
The Toolbar at the top can be used to navigate to the previous or next page, set the zoom setting to width, zoom in or out. Use the selection tool to only select content as text or tables (creates tables) or an entire area, and draw a box around the content that you wish to copy.
The View menu has a full-screen mode for a semi-distraction free reading. The presentation mode is full-screen too, but it only lets you switch pages, so many options like zooming in/out aren't available. Speaking of, the zoom controls are available in the same menu.
The Continuous mode is useful for smooth scrolling through pages. The View Modes which includes Facing Pages, Overview and more are useful when you want to read eBooks in a 2 page view. You can also trim the view to margins, or draw a rectangle around the portion of the page to trim the rest away. Got a weird document that's in landscape orientation? Okular has a rotate tool that can help you change it to portrait mode, and vice-versa.
The Edit menu has the basic copy, select, find tools. The Go menu has options to navigate through documents. The Bookmarks option allows you to manage saved bookmarks, rename them or remove them. The Tools menu has some extra options such as the magnifier tool, and more importantly the annotation tool, which is called "Review". It allows you to add a note as pop-up, inline, highlight text, add a stamp, or to draw a line (freehand/straight), polygon, ellipse or a cool Typewriter annotation.
Okular's interface is highly customizable, you can add/remove content from the toolbar, hide the page bar, menu bar, side panel, scroll bars, etc. Go to the program's options for more settings for annotations, presentation, editor, etc.
Calling Okular a document viewer is an understatement, because it can also be used to view images, though that isn't its primary function. Here are all document and image formats that are supported: G3, CHM, DDS, DJVU, DJV, EPS, EPSI, EPSF, EXR, FB2, GIF, XCF, HDR, PIC, JPEG, JPG, JPE, JP2, JPG2, MNG, MOBI, PRC, ODT, OKULAR, PBM, PCX, PDF, PGM, PNG, PPM, PS, PSD, RGB, TGA, ICB, TPIC, VDA, VST, TIF, TIFF, DVI, WWF, BMP, DIB, ICO, XBM, XPM, OXPS, XPS, CBZ, CB7, CBR, CBT, EPUB, DOC
Though not listed, it does open TXT Text documents. The application can export documents in the DOC plain text format.
I installed Oklular from Snap on Linux Mint. You can do so with the command
sudo snap install okular
The Windows version (from Binary Factory) comes in a 63MB portable package, but it actually unwraps to about 320MB. The installer version weighs exactly the same. But, the description of the Windows Store version which was released a couple of months ago, says its 477MB. I'm not really sure why the size differs so much, it probably has something to do with Windows Store dependencies.
The interface of the Linux and Windows versions are the same. I found some minor differences like the Full-screen mode available in a different menu (Linux > Settings, Windows > View).