I have a similar problem in that I can't open a PDF file from the email client on my iPad.
Even if I ask to open in iBooks, all I get is hours of spinning wheel.......
BUT if I use safari and open the files I can preview and load into iBooks.
Try using safari and go into hotmail via the web and see if the files are there and will open, like I did.
If they do we need to look into why they won't open from the email client
Andy