These permissions are set by library.
You can check by following below steps:
Please check the following:
1. Open the file in Adobe Digital Editions.
2. Go to File -> Item Info
Check under permissions, Viewing permission
This didn't help me, I got a popup saying "this document is registered to a different user" (or something similar).
I think I found a solution! When downloading from the library, and it asks you if you want to Open with Adobe or Save File, which I always do out of habit, I chose Save File, and it loaded to my Nook-and opened, much to my surprise. So I tried loading another library book to my Nook using the same method and it opened in my Nook, too.