FAQ Information.

When you start to download a protected Adobe PDF or ePUB a small file with the extension .acsm is first downloaded. This is used by Adobe Digital Editions (ADE) to send the activation ID to the delivery server which will use that ID to generate an encrypted PDF or ePUB eBook, which is then downloaded to your PC. 

Authorizing 

If your computer is asking you where to save the .acsm file then your ADE may not be installed correctly or not yet authorized. Often you can solve this issue by simply setting your computer to always use ADE to open this type of file (in Windows XP, for instance, right click on the file and choose Open With, select Adobe Digital Editions (ADE) and tick the box to tell Windows to remember the setting; for other operating systems please read your documentation). 

Many eBooks have built in security, Digital Rights Management (DRM), in order to protect the eBooks copyright. You are required to activate your software before downloading to identify that you are the rightful owner of the eBook. In the case of ADE you can opt not to authorize the software, but if you do this you will not be able to transfer the eBook(s) to other devices you own and in the event of having to reinstall ADE it may not be able to open your existing eBook(s). 

Again, when you download an encrypted DRM file such as one from a library you might see a .acsm file instead of the file you wanted. Generally this is invisible if everything is set up correctly. 

The .acsm file contains the encryption codes necessary for downloading the actual eBook. All you should need to do is open the .acsm file using Adobe Digital Editions (ADE). Usually, when you install ADE it tells your web Browser to do this automatically when it sees an .acsm file."