title_body: How to confirm if ePub is DRM'ed ? Kobo Aura HD possibly refusing to read DRM-free ePub
upvoted_answer: I have a different type of Kobo (Glo HD), and no knowledge of DRM
  implementation for ePub, though I would expect that a variety of
  distinct DRM techniques can be used on ePub files.
  
  I would however expect that an eBook from O'Reilly does not carry DRM. So your problem should have another cause. Which you apparently confirmed by testing on different devices or application software.
  
  What I noticed by experimenting with my Kobo is that, when an eBook
  has been loaded and made visible to the Kobo, it may interpret reading
  problems as caused by the presence of DRM, though that may not be the
  case. And you get an error message that pretends that the book is DRM
  protected.
  
  Typically that can happen if some mishap destroys the file or
  moves it to another directory in the Kobo memory. That is easily done
  by accessing the memory directly, as done by tools such as Calibre,
  though this should in no way be construed as suggesting that Calibre
  makes such mistakes.
  
  From what I understood experimentally (so I may be wrong) The Kobo
  believes it has a DRM problem with that eBook until it does a scan of
  its memory, and refreshes its tables, which will then no longer point to a
  missing ePub file. I am not sure what is the best way to force that scan.
  The way I did it was by using the Kobo browser to download in the Kobo a DRM-free
  book (I usually go to http://gutenberg.org for that).
  
  So it could be that your Kobo has some problem reading the file, for
  whatever reason, and interprets it as a DRM problem, exactly as it did
  for my missing/displaced files.
  
  My idea is not to give you a definite answer on what is wrong, as only
  you can do that. It is more to suggest that the Kobo may interpret as
  a DRM problem a problem unrelated to DRM.