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As others have said, one should assume that there is some identifying information which survives DRM and even format conversion. Even if you were to take the trouble of converting to plain ascii text and painstakingly review the file you could still not be certain. There are so many different techniques developed over many years to detect the source of leaked versions which have potential application. For instance, circulating a paper using different words or phrases in different places so as to make each unique. This would not be detectable even theoretically without other copies of the work to test against. Even then it may prove difficult. And this is but one approach.
It is naive to assume that those who take the trouble to use DRM will put all their eggs in one basket. Eliminating any risk is both labour intensive and unreliable. One could never be sure that the book has been totally sanitised.
The assumption that this is not happening because no one has so far to our knowledge been prosecuted after being detected through such information is also an unwise one. There are many reasons for not resorting to legal action.
And, of course, even if such techniques are not being utilised now, they can be implemented at any time. I recall one of Fred Saberhagen's Berserker short stories made the point quite nicely. Whether a man's life was forfeit for treason was determined by a fake entry placed in the ubiquitous Galactic Encyclopedia to catch copiers.
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