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Most purchased EPUB books have DRM. This prevents calibre from opening them.You can still use calibre to store and transfer them to your ebook reader. First, you must authorize your reader on a windows machine with Adobe Digital Editions. Once this is done, EPUB books transferred with calibre will work fine on your reader. When you purchase an epub book from a website, you will get an ”.acsm” file. This file should be opened with Adobe Digital Editions, which will then download the actual ”.epub” ebook. The ebook file will be stored in the folder “My Digital Editions”, from where you can add it to calibre.