Sumatra PDF is a free PDF, eBook (ePub, Mobi), XPS, DjVu, CHM, Comic Book (CBZ and CBR) reader for Windows.
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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 Free and open source
Sumatra is available for free, licensed under GPL with source code available on GitHub.
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
It's available in 69 languages.
Pro Elegant interface
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.