I can't check for sure but if you have an amazon account you should be able to buy the ebooks. I know they have it in sets here and here. It's kind of pricey. I also know b&n carries the ebooks. I know google books has it as well, but it's more expensive. You can get it through the play store on android.
As for downloads & such I am not sure. If you do google to read pretty little liars online some stuff pops up. You can try and torrent, but that's pretty risk anymore now with the better sites down.