An ACSM file is a message file served by the Adobe Content Server to Adobe Digital Editions, a program used to manage and read eBooks as well as other digital publications. It contains data to activate and download an eBook but does not contain the eBook itself. ACSM files are used to authorize, legally download, and read purchased eBooks in Digital Editions.
Most Digital Editions users will not encounter an ACSM file. If you do find an ACSM file, it most likely means that the download of your book was interrupted. You can double-click the ACSM file to resume the download and open the file in Digital Editions. The ACSM file will be located wherever your browser saves downloaded files.
NOTE: Even if you have a different eReader software other than Digital Editions, such as Kobo Desktop, you may still receive an ACSM file when downloading an eBook. However, you still must install Adobe Digital Editions to download the eBook and then it can be read and transferred to your eReader device.