There is a known incompatibility issue between Kobo e-readers and certain ebooks protected by the Adobe DRM.
This issue causes the transfer of an ebook in EPUB 3 format from Adobe Digital Editions version 4 or 4.5 to any Kobo e-reader to fail, and displays an error message that pretends that the book is DRM protected.
It turns out that an attribute change to the protection settings made by these versions of Adobe Digital Editions is not supported by Kobo e-readers, resulting in the EPUB file not opening on these e-readers.
The best way to avoid this problem is to use the up-to-date Adobe Digital Editions version 4.5.12.
In the following articles you will find how to install Adobe Digital Editions version 4.5.12, or uninstall vesrions 4 or 4.5 on your PC or on your Mac.