So far as I know the two largest distributors of SF Ebooks, TOR and Baen, do not use DRM on their ebooks and Tor reported after a year that they saw no difference in pirating before and after dropping DRM.
https://www.tor.com/2013/04/29/tor-b...ne-year-later/
IMO DRM is a PITA for readers who want control of their book files and, as posted here, discourages purchases from some potential buyers. Besides it is apparently readily bypassed by free software by any one who really wants to.