In what format are books being shared? .pdf? .mobi? .awz?

There's some kind of new site called something like "Lendle", where Kindle owners can share their books with other Kindle owners. I don't know if this gets around the requirement of the publisher's approval for a book to be lent, but I think it's a really worrying development. A whole, central site, presenting itself as cheerily legit, devoted to sharing Kindle files, could be devastating.