Starting with the release of the PMBOK Guide, 6th Edition, PMI has updated their terms to not allow the printing of the PMBOK Guide PDF version. The 'Print' feature has been disabled in the PDF version.
According to PMI, this has been done to tackle piracy.
You can read more about this change here:
Why can’t I Print the PMBOK Guide, 6th Edition PDF?
What do you think about this change?