Read Online will let you scroll through the e-book and read it from your browser. Generally, this is the fastest and easiest way to read an e-book.
Download Books or Full Download allows you to download the entire e-book through Adobe Digital Editions. You will need to check the e-book out for a certain number of days, and the e-book will expire from your Adobe Digital Editions reader when the time is up. You can check the e-book out again if you need more time.
Print options are useful if you are only interested in a small portion of the e-book - a chapter, for instance. Due to copyright restrictions and publisher rules, the number of pages you can save or print is usually limited to a certain amount of pages per day. Some e-books may not offer this option at all.
Download PDF option lets you download a chapter into a PDF and save it to your computer. This is useful when there is a limit to how many users can access an e-book simultaneously. It also saves you time, so you don't have to open the e-book each time you want to read a chapter. Keep in mind each e-book has a limit to how many pages you can download at one time. This limit resets every 24 hours.