With Amazon.com not selling Macmillian books at the moment, now might seem like a good time to go to the source and buy ebooks directly from Macmillian. And they even have multiple formats available!: the “Adobe Digital Edition” format and the “Adobe eReader” format.
According to their Ebooks information and help page, “Adobe eReader” books enable you “to read high-fidelity ebooks alongside other PDF files. Only this reader software displays ebooks with the pictures, graphics, and rich fonts you’ve come to expect from printed books.” While “Adobe Digital Editions” is “Adobe’s reader designed for eBooks” and “uses a format based on the Open Publishing Standard with the extension .epub,” but “ADE will also display your PDF files in a double-page, single page, or fit-to-width view — or you can specify your own custom fit.” Both formats have software download links, which both redirect to Adobe’s current Digital Editions page.
So. Er. I think “Adobe eReader” is PDF and “Adobe Digital Editions” is EPUB. Format proliferation is bad enough without making format identification more difficult than necessary.