Downloaded book from library visible on my computer, copy file from my downloads to my Kobo ereader, and get an error message that this document is protected by Adobe Digital Rights Management and is not currently authorized for use with your Adobe ID.
We apologize for the delay in response.
Seems the downloaded file(ebook) was created using the Adobe Digital Edition, and Kobo ereader has not been authorized to use with the same Adobe i.d which was used to create the ebook(downloaded file).
You may refer to the "helpful" answers of forum thread where similar issue has been addressed- Oops! This document couldn't be opened.