Yup! It's free on Uplay. Once you download it, it's yours to keep so it's not something that's limited for a short period. 
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Edit:
Just an update for anybody who's interested - it's still available, from 18th to 23rd December now. World in Conflict complete edition (classic 2007 RTT/RTS game) and Watch Dogs (middling action/adventure game from 2014) are also available for free during the same period. They'd already given World in Conflict away earlier this month before the original AC4 offer started.
https://www.ubisoft.com/en-gb/event/happy-playdays-2017/
As the offer isn't valid anymore, mods can delete or lock this thread.
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