Error message: "Adobe Reader could not open because it is either not a supported file type or because the file has been damaged (for example) it was sent as an email attachment and wasn't correctly decoded."
This is known as the ACSM or file association issue.
Sometimes a "file association" error occurs in Windows. What this means is that Adobe Digital Editions (ADE) is not set to open ACSM files, which are used to download eBooks. If you can't open eBooks in ADE, follow the appropriate steps below.
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ACSM should only be there if you've installed an application that can handle .acsm files or if you've tried to open an .acsm file and told it to use always use an existing application. It's also possible that the association existed in the past but was deleted somehow.
If your computer simply says it does not know what to do with .acsm files then you should get a prompt to find a program either online, or in the store, or already existing on this computer (the wording, options, and whether they are shown right away or you have to follow a "more options" link vary by Windows version). If you already have Adobe Digital Editions installed, then use the option that corresponds with "find existing program already installed" and then browse to the DigitalEditions (or DigitalEditions.exe) file in \program files\adobe\adobe digital editions\ (or with the (x86) if present).
For exact instructions on how to make the change on your version of Windows try performing a Google search for "tell windows 10 to alway open extention using specific program" or similar. Replace the 10 in the search with your version of windows.