eBook is the short form for electronic book. Most popular formats are Adobe's PDF (.pdf file extension) and Microsoft Reader (.lit file extension).
Our choice is PDF. Invented by Adobe Systems and perfected over 15 years, Adobe Portable Document Format (PDF) lets you capture and view robust information—from any application, on any computer system—and share it with anyone around the world.
Individuals, businesses, and government agencies everywhere trust and rely on Adobe® PDF to communicate their ideas and vision.
(source:
http://www.adobe.com/products/acrobat/adobepdf.html)
The PDFs you buy from us are non-DRM (do not come with a Digital Rights Management) mainly because DRM quickly becomes a burden for the client, not allowing you to copy and access the file to another computer or personal device.