PDF is a popular document sharing format that has been around since the early 90s. It is highly formatted, and does not work well with smaller devices.
Devices: Works well with Sony Reader, Amazon Kindle, Barnes & Noble Nook, the iPad, and the Kobo eReader, and less so with the iPod Touch.
Collections: PDF is used in virtually every academic eBook collection that allows downloads.
In the case of e-books, DRM typically restricts which devices and programs can view the e-book, if the e-book can be printed, and whether the e-book can be downloaded and transferred between devices as a file or if it must be viewed in a specific browser or reading program.
For more information about DRM:
Epub is an open eBook standard that provides reflowable text, inline images.
Devices: All ereaders, except for the Kindle, support this format.
Collections: Epub is used less frequently with academic eBook platforms than other environments. However you'll be able to find epub files on the following major publishers/platforms:
DRM: PDFs are often accompanied by Adobe Digital Editions DRM software which may contain additional restrictions impacting how you can view or download eBooks.
Full-text academic books can be viewed online via various browser based eBook vendor platforms. These platforms require internet access to load the books however some will also allow offline reading in the browser after the book has been loaded. Check your mobile browser settings to see if offline browsing is available.