Download version 2.01 instead of 3.x or 4.x.
Version 2.01 only supports the old Adobe DRM, which has been cracked in 2009. Version 3.x and 4.x support newer versions of DRM, which have not been cracked at this time.
If you use ADE version 2.01, your book will always be encrypted with the old DRM version, which you can remove by using Apprentice Alf's tools. If you use ADE version 3.x or 4.x, there is a chance that your book is downloaded with the new DRM, and then you can not remove it at this time.