What is ACSM? It’s sort of a “key” file for EPUB or PDF eBook, especially for an eBook sold by some online book store or online library. With this “key”, you can get the correct EPUB or PDF eBook associated with “key”. So, don’t be worry next time you buy a book from online book store but only get an ACSM file, with it you can download a complete book. To read ACSM file, you only need one tool: Adobe Digital Editions. Below is the instruction.