I have been running ADE on my Macbook for a couple of years now, and have no trouble syncing/copying books between my Kobo e-reader and ADE on my Macbook.
I recently bought a new iPad and have installed the ADE app on the iPad. I succeeded in authorizing my iPad with my Adobe ID (I had to reset the password because I've lost the old one, it's been so long since I needed to access it). I have set Auto Download as Enabled under Settings in ADE app.
How to I copy the e-books over from my Macbook to my iPad, in ADE? I have tried opening my iPad in iTunes and then tried copying the e-books either from ADE on my Macbook into iTunes (doesn't work), also opened Finder on my Macbook, went to Documents>My Digital Editions and tried copying into iPad via iTunes (doesn't work). I ejected my iPad from iTunes and re-attached it to my Macbook after launching ADE on my Macbook, in hopes that ADE on the Macbook would recognize my iPad and I could then copy over the files to my iPad from within ADE on the Macbook (which is what I do to copy books onto my Kobo e-reader after I've purchased them, got the acsm link and used that to download into ADE, then attached the Kobo to my Macbook and ADE recognizes it and I can copy over into the Kobo. Doesn't work with my iPad.)
So, I've authorized ADE on my same account on both my Macbook (where ADE has my entire epub book collection) and also authorized the app on my iPad. How do I get the epub files from my Macbook over to my iPad's ADE app?
After posting this query, I spotted another query and answer from about a year ago, which solved the problem.
My problem was that the feature in iTunes referenced in the above link, is something I (and probably a lot of other iTunes users) never knew about, or forgot about because I never needed to use it until now.
My one quibble is in the documentation for the Adobe Digital Editions app. The documentation, which installs with the app, does mention that you can move e-books from ADE on another device (my Macbook) into the app on an iOS device, but it isn't at all clear in that documentation how this is done. Certainly not as clear as the instructions posted by a helpful Adobe staffer in the above link, which are very clear. May I suggest, Adobe, that you update that documentation by including a cut-and-paste of the above directions?
The documentation mentions a couple of other ways you can move ebooks over to iOS without using iTunes, but in my case (more than 200 ebooks), all those other methods would be incredibly time-consuming and tedious. The iTunes trick mentioned above works beautifully and very quickly; I didn't time the operation, but I don't think it took more than 3-4 minutes to move my whole collection over onto my iPad.
I thought I should post this reply, in hopes a) of attracting attention from someone at Adobe who could improve on that documentation for the app, and b) in case other users stumble onto my original query without having see the reference above.