A file with the .ACSM file extension is an Adobe Content Server Message file. It's used by Adobe Digital Editions (ADE) to activate and download Adobe DRM protected content.
It's important to realize that ACSM files are not e-book files in the regular sense; they can not be opened and read like other e-book formats, like an EPUB or PDF. In fact, the ACSM file itself is nothing but information that communicates with Adobe's servers.
There isn't an e-book "locked inside" the ACSM file nor is there a way to extract the book from the ACSM file.
Instead, ACSM files contain data from the Adobe Content Server that's used to authorize that the book has been legally purchased so that the real e-book file can be downloaded to your computer through the Adobe Digital Editions program, and then read back through the same software on any of your devices.
In other words, once your device is properly setup, you can open an ACSM file to register the book to the ID you've configured Adobe Digital Editions with, and then read the book on any device that's running ADE with the same user ID, without having to repurchase it. There's more information on that process below.
How to Open ACSM Files
Adobe Digital Editions is used to open ACSM files on Windows, macOS, Android, and iOS devices. When the book is downloaded on one device, the same book can be downloaded to any other device that's using Adobe Digital Editions under that same user ID.
Note: You might be asked to install Norton Security Scan or some other unrelated program during the ADE setup. You can opt out of it if you wish, just be sure to watch for that option during the installation.
You have to use the Help > Authorize Computer... menu option in Adobe Digital Editions in order to connect your e-book vendor account to Adobe Digital Editions.
This is the only way you can be sure that your books are available on your other devices, that they're re-downloadable should your device fail or the book is deleted, and that you don't have to buy the book again for your other devices.
Once you've done that, you can only read Adobe DRM protected data that you've been authorized to through the account you entered on that authorization screen. This means you can open the same ACSM file on other computers and devices too, but only if the same user ID is being used in Adobe Digital Editions.
Note: You can also authorize the computer without an ID by checking the appropriate box through the Authorize Your Computer screen.
How to Convert an ACSM File
Since an ACSM file is not an e-book, it can not be converted to another e-book format like PDF, EPUB, etc. The ACSM file is just a simple text file that describes how to download the real e-book, which might in fact be a PDF, etc.
Due to the DRM protection, this probably won't work, but you might have luck converting the actual e-book file to a new format. Find the file that was downloaded through Adobe Digital Editions and open it in a file converter program that supports the format that the book is in, like Zamzar or Calibre.
From there, convert it to a format suitable for your needs, like AZW3 if you want to to use the e-book on your Kindle device.
Tip: To find the book that ADE downloaded using the ACSM file, right-click the book in Adobe Digital Editions and choose Show File in Explorer. In Windows, this is most likely in the C:\Users\[username]\Documents\My Digital Editions\ folder.
Still Can't Open Your File?
Since it's a bit different than other file formats, if you can't open your ACSM file, make sure to take note of any errors you see. If there's an authentication error when opening the e-book, chances are that you're not logged in under the same ID that purchased the book or you don't have ADE installed.
However, if you've done everything right and your file still isn't opening with the suggestions from above, double-check the file extension to make sure it actually reads "ACSM." Some file formats use a file extension that's spelled similar to ACSM but is in fact different and therefore uses different programs to open the file.
For example, ACS files are Agent Character files used with Microsoft Agent. Though the file extension is spelled almost exactly like ACSM, it has nothing to do with Adobe Digital Editions or e-books in general.
Another similar file extension is ASCS, which is reserved for ActionScript Communication Server files. Even though they are used by an Adobe program, Adobe Device Central, they also have nothing to do with e-books or ADE.
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