The Adobe Content Server (ACS) digital lock is Adobe's solution for rights-management. It applies to both PDF and EPUB files. In order to access an ebook protected by ACS, you must first obtain a free personal identifier (Adobe ID), by creating an Adobe account, and then install one of the reading applications that are compatible with ACS (for example Adobe Digital Editions).
As soon as a user opens the downloaded file for the first time on his device, a licence is automatically granted to this user's Adobe ID, and for this device. This licence limits activation to 6 different devices, always using the same Adobe ID.