Free PDF Reader - Sumatra PDF by Krzysztof Kowalczyk
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Sumatra PDF is a free PDF, XPS, DjVu, CBZ and CBR reader for Windows.

Sumatra PDF is small and starts up very fast.

Simplicity of the user interface has a high priority.

Portable version included (a single executable that can be run e.g. from USB drive and doesn't write to registry).