When comparing FBReader vs KDE Okular, the Slant community recommends KDE Okular for most people. In the question“What are the best desktop readers?” KDE Okular is ranked 3rd while FBReader is ranked 6th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Built in eBook store
FBReader features an in built eBook store for users to download eBooks onto their devices through the app.
Pro Simple and fast
FBReader, while offering many options is simple in design, with that allowing for fast use.
Pro Books can be saved to SD card
Pro Cloud sync
FBReader has built in Google Drive sync for the users files, allowing them to store their eBooks online, not taking up room on their local storage.
Pro Free and open source
Pro Trim margins
Easily trim margins either automatically or manually for easier reading
Pro Featureful
Pro Table selection
Pro Tabbed view option
Pro Supports touch interaction
Cons
Con Cannot search for books on a SD card
Con No light mode
Unusable if you have astigmatism.
Con UI is bad
User interface is confusing and ugly.
Con Complex and confusing app settings
Con No XFA Adobe Forms support
Cannot fill PDF Forms created with Adobe.
Con Requires many KDE libraries
Con No middle-mouse auto-scrolling
Instead of scrolling automatically when holding down the mouse wheel and dragging, it instead zooms in or out, in contrast with many other programs.
Con Poor HiDPI support
You may have to tinker with QT__SCALE_FACTOR environment variables to get the desired size and not blurry content (this is a bug; see https://bugs.kde.org/show_bug.cgi?id=362856 and https://bugs.archlinux.org/task/54688)
Con No search results overview
You have to navigate to next/previous hit one by one
Con No "fit to page" option under printing settings
You can only print the content as given so, when you receive a bigger or smaller image thant the default of the printer, you will have to edit it first on other editor.
Con Slow scroll
Its new ultra-slow-scroll for PgUp and PgDn makes it unuseable.