That will depend on where you are downloading then book from.
I have never been asked for such a code from libraries or other places I have bought books.
The 'unlocking' is generally done using your Adobe ID in that case.
Your book vendors may have used Adobe DRM but with their own vendor id.
They will be able to advise you what the id and password to use are.
If the book is PASSHASH enabled then it will prompt for Unlock Code. Please check with your distributor and get the login credentials to unlock.
For more information on PASSHASH refer to, Public Knowledge Base
Hope this solves your problem.