eBook? What exactly am I buying?
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[Adobe] [Wikipedia: Portable Document Format]
How do I read/open it?
By 2020 most modern devices have a built-in method for handling these files.
If you require software, there is plenty of free apps/programs to read PDFs with. PC users try
Foxit or
Sumatra.
Mac users try Skim. Unix/Linux users, you know what to do. Apple iPhone users → Apple Store. Android → Playstore.