How does the library know when my borrowing time is over?

The PDF files contain metadata which is handled by Adobe Digital Editions. Metadata is necessary because of the terms of the lending and copyright agreement.

This program enforces Digital Rights Management (DRM) on the downloaded files, restricting the time period during which the ebook can be used, as well as the number of computers (Reviewbites) to which it can be transferred; there may also be restrictions on the number of pages that can be printed.