ACSM stands for Adobe Content Server Manager. These files are part of Adobe's DRM ebook download process. Adobe provides DRM (digital rights management) for book publishers to protect their ebook files from copyright infringement and illegal distribution. When you buy and download a PDF or EPUB ebook that is protected by this DRM, an .acsm file will be involved in the download process.

Most users will not even see this file. ACSM files are made to operate in the background of your ebook download. These files manage the communication between your computer and Adobe's content server. Generally, the only time you will see the ACSM file is if you don't have the correct software installed. When you do have the correct software installed, the ACSM file will do its job and manage the download of your ebook.

The program that you need is called Adobe Digital Editions. You can get it from Adobe's website for free. If your computer does not have this program installed, it won't know what to do with the ACSM file, and your ebook download won't work. You'll end up with a small file that ends in .acsm and your computer won't know how to open it. This is a normal situation. Anytime you have a file on your computer and you don't have the program for it installed, your computer won't be able to open the file. For example, if you have a .psd file but you don't have Photoshop installed on your computer, you won't be able to open that file.

If you have an ACSM file but no way to open it, then all you have to do is install the program that you need. Once you install Adobe Digital Editions, you can open the ACSM file in that program, and your ebook download will continue. Once the download is finished, you will have the PDF or EPUB file that you expected.