When you buy an eBook it can have DRM protection, you could open and read it only with propriety software.
Calibre: to manage your comics and mangas
When you buy an eBook it can have Adobe DRM protection: a small file with standard name
URLLink.acsm. Basically, it's a file for Adobe Digital Editions software, it can be downloaded by this application and read only by this application. To erase it, you need to download Adobe Digital Editions to download eBooks, install it and open
URLLink.acsm, Adobe Digital Editions will download the eBook and when it's done, you can click right on the eBook to choose Display file in explorer, you will find an EPUB file under protection.
You need to have Calibre and DeDRM plugin (download last version DeDRM_tools.zip), when Calibre it installed go to Preferences->Plug-ins, select Load plug-in from file and find DeDRM zip file that you downloaded. Install it and import EPUB file from Adobe file location, the DRM will be erased.
Source: Remove Adobe DRM article