Sumatra PDF is a free PDF, eBook (ePub, Mobi), XPS, DjVu, CHM, Comic Book (CBZ and CBR) reader for Windows.
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Pro Free and open source
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
It's available in 69 languages.
Pro Low resource usage
Pro Elegant interface
Pro Extensive keyboard shortcuts
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.