What is the antonym of DRM-free?

Solution 1:

The most common term seems to be DRM protected. Other relatively common terms, in approximate descending order, include DRM locked, DRM enabled, DRMed, and DRM encumbered.

A few notes:

DRM protected seems to be relatively neutral, whereas DRM locked, DRMed, and DRM encumbered seem to be slightly pejorative and DRM enabled seems to be slightly, um, ameliorative.
DRM enabled is a bit ambiguous, in that it's also used to describe devices that support DRM. But the intended meaning is usually clear from context.
As Martin Smith mentions above, DRM can take a number of different forms, and doesn't necessarily prevent downloading.

Solution 2:

"DRM-locked", especially if you're looking for a single word similar to "DRM-free". For example, Kindle books, which are under DRM, will give an error saying "This book is locked by DRM" or something similar when you attempt to edit them with an ebook-managing program.

In fact, "under DRM" also works if there are instances where a single word doesn't sound right.

"DRM-limited" is also an option, but it's less clearly an antonym of "DRM-free" and it's similar enough in form to the better "DRM-locked" that there's probably no need to ever use it.

Solution 3:

Copy Protected

I've seen this definition for it:

A class of methods for preventing incompetent pirates from stealing software and legitimate customers from using it. Considered silly.

The New Hacker's Dictionary, third edition, by Eric S. Raymond