I'm assuming Windows not Mac. There must me a similar thing on Mac, but not sure of the details.
- Find a .acsm file in Windows Explorer.
- Right click on it.
- Choose 'Open With'
- select 'Choose Default Program'
- find Adobe Digital Editions.
- If it is not on the list, you will need 'browser for application' or similar (varies with Windows Version). Probably at
C:\Program Files\Adobe\Adobe Digital Editions 2.0\DigitalEditions.exe - Click OK a couple of times.
At least one person recently had problems with this (on Windows 7 I think), which just consistently ignored the instructions.
The more recent versions of Windows do seem to have a bug that sometimes makes setting up the default OpenWith unreliable.
Alternatively, you could also try reinstalling Adobe Digital Editions (probably no need to uninstall first).
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If you can't get filetype registration to work for automatic open, you can open .acsm files in ADE by drag-drop from Explorer.
(because of bad ADE design, ctrl-O open won't work for .acsm files)