I cannot view the downloaded e-book in my PDF reader or EPUB reader
DRM-protected e-books cannot be read in any PDF or EPUB reader. You need a program that can open and display DRM-protected books. On a PC or notebook, use Adobe Digital Editions, which you can download from the Adobe website. For use on the tablet and smartphone, there is the option to switch to reading software from Bluefire, for example. This is available free of charge from the App Store / Play Store.