Adobe Digital Editions (ADE) Is an app that is required to for you to read ebooks that you have downloaded to your device. Book publishers use digital rights management (DRM) to restrict access to ebooks and control the length of time that you are allowed to read a downloaded ebook. This is enforced by having ADE as the app needed to open ebooks, as ADE supports DRM.
If you're not comfortable using apps, then perhaps it might be easier for you to read books online instead of downloading them
Further Reading
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Installing Adobe Digital Editions
