Most times people say this it is because they've deleted the app and reinstalled it, or are on a new device. Note that the Adobe ID account is simply to be compatible with Adobe's ebook copy protection tech (DRM) and that the ebook files themselves are in no way stored in the cloud based on your Adobe ID. So, if you delete the app/books, or get a new device, authorizing the app with your Adobe ID will NOT somehow magically restore your ebook files. I recommend you back them up by exporting them to a Cloud storage location (like Dropbox or iCloud). To do this, select all the files in your library (inside the app) and then tap on the "share" button (looks like a box with an upward facing arrow). Then in the Files app, navigate to a cloud storage location and save them there. You can also import files from cloud storage by tapping on the "+" button in the library view of the app.
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