Kobo Books Query

I've just acquired my first books from the Kobo book store, two were free and one I bought and they arrived shortly after on my Auro.

I do like to backup my books and it was then I had a bit of a problem. From the list of my books on the Kobo website I clicked to download them all. Two books, one free and one I purchased came as .acsm files and the other an epub. The epub I could open easily but Adobe Editions couldn't open the other two.

I am struggling to understand why this should be. Do other people have this same problem?

Thanks.