This means that the ebook was originally a PDF and has been encrypted to protect the author from the risk of piracy.
PDF is the most popular document publishing format for general use. You’re probably familiar with PDF – it’s the file format that normally opens up in Adobe Reader or Adobe Acrobat. In order to give users a simpler and “cleaner”, less cluttered reading interface, Adobe developed Digital Editions, especially for reading ebooks. Most recognized book publishers release ebooks in "copy-protected", encrypted form and PDF for DE is a popular choice.
The great thing about viewing PDF ebooks is that they retain the original design of each page – so you see on your screen exactly what the book designer intended. Formats that are more suited to smaller screens (like ePub) tend to lose page formatting when displayed on a computer screen.
Adobe Digital Editions has been developed to make it possible for you to read your favourite ebooks on computers as well as a wide variety of hand-held devices, phones and ebook reading devices. The ePub format is perfect for reading on smaller screens, because the lines of text 'reflow' and adjust to accommodate varying font sizes and screen sizes.
The iPhone / iPad / iPod Touch format is designed to be read using eBooks.com's Ebook Reader app, available from the iTunes App Store (link). This format can be purchased, downloaded and opened in seconds using either of these devices, anywhere that you have access to the internet.