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:

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).