When comparing Sumatra PDF vs Okular, the Slant community recommends Okular for most people. In the question“What are the best PDF readers?” Okular is ranked 2nd while Sumatra PDF is ranked 3rd. The most important reason people chose Okular is:
Easily trim margins either automatically or manually for easier reading
Question Ranking
Pros
Pro Free and open source
Sumatra is available for free, licensed under GPL with source code available on GitHub.
Pro Portable
The application consists of a single file that can be run without installation. You can put it on an external device, such as an usb thumb stick and run it on any Windows machine from the thumb stick.
Pro Starts quickly
Pro Extensive keyboard shortcuts
Pro Does not have heavy editor features
Annotations and other heavy features are just not needed in reader.
Pro Low resource usage
Pro Multilingual
It's available in 69 languages.
Pro Elegant interface
Pro Trim margins
Easily trim margins either automatically or manually for easier reading
Pro Free and open source
Pro Supports touch interaction
Pro Table selection
Cons
Con Lacking in features
Sumatra aims for simplicity at the expense of functionality. For example, there's no way to annotate PDFs or edit them.
Con Windows only
Con Fonts are not smooth enough
Not as smooth as e.g. Adobe Reader. No anti-aliasing
Con Cannot crop empty fields
Just got used to this killing feature on Goodreader on Mac.
Con Requires many KDE libraries
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 "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 No XFA Adobe Forms support
Cannot fill PDF Forms created with Adobe.