just downloaded 5 free e-books (in PDF) from Wowio.com

I got 5 Kurt Vonnegut books for free!

Only problem is, when I try to open a book with the native pdf reader, I get this message:

Unable to open. File format not supported.

On my PC the properties of the files say they Adobe Acrobat 7.0 documents.
I have opened another pdf file (not a book) that also says it's a Adobe Acrobat 7.0 document.

Any idea why I can't open these books?
Neil