Most Adobe ebooks have DRM (encryption), but are still readable on up to 6 PCs and 6 devices (such as a dedicated ebook reader). The limitation is that all need to be authorized to the same AdobeID, and you need to read them using Adobe provided or licensed software.
If you remove the DRM, then these limitations go away. This is probably legal in the US for personal use of ebooks you bought. See
Adobe ADEPT DRM for EPUB circumvented (Adobe PDFs are also circumventable with a similar script from the same source). Be sure to read the blog comments for the latest news.