[any ANONYMOUS forum user]

If it is an epub file with Adobe Digital editions DRM, then you must use a program to determine your adeptkey.der file (after registering with Adobe Digitial Editions website) and once you have that you can remove the drm.

Download ineptkey.pyw and ineptepub.pyw and use them as mentioned above. Many DRM epubs have faulty zip archives built so you might need to fix that as well.