Owners of the physical version can download the digital vesion for free.
The DRM-free Microsoft Windows PC versions of the "S.T.A.L.K.E.R." franchise is on sale for 60 percent off on GOG until just before 9:00 AM PST on January 30, 2017:
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S.T.A.L.K.E.R.: Shadow of Chernobyl - $7.99.
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S.T.A.L.K.E.R.: Clear Sky - $3.99.
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S.T.A.L.K.E.R.: Call of Pripyat - $7.99.
Important note: If you already own a physical or Steam copy of those titles -- you can claim a free digital copy on GOG! You just need the serial number from your physical copy (it's printed on the instructions).