can't open .acsm file

Downloaded a library book to the hard drive.  When I tried to open it in Digital Edition 2.0 (i need 2.0 because it is a first generation Nook) got the following error: "Adobe Acrobat DC could not open "[title of book] .acsm" because it is either not a supported file type or because the file has been damaged."   Don't care why or how this happened: just want to fix it.  Help!