I can open an ebook on my Mac desktop, but it won't open on my iPad.

[#1]

During the ADE process, I was assured that once I had an ebook, I could read it on ALL my devices.  It will open on the desktop.  It is in the library on my ipad, but it won't open.

[#2]

During the ADE process, I was assured that once I had an ebook, I could read it on ALL my devices.  It will open on the desktop.  It is in the library on my ipad, but it won't open.