Some e-books sold on internet can be delivered as .acsm files, which means that they work only with the Adobe digital platform. These files actually cannot be converted to any other electronic book format (such as pdf in this case), because .acsm files do not represents the actual e-books. They are just authorized links so the user can download and view legally purchased books on their computers.
The only way to "convert" acsm to pdf would be to install Adobe Digital Editions and authorize your computer with your Adobe ID, then open the .acsm file and load it in the program. After the book is downloaded, you can find it either in PDF or ePUB form in the default Adobe Edition location in user account and documents. Sometimes you also need to remove the DRM protection from the resulting .pdf document, because these files are still protected by Adobe DRM which sometimes cannot be read on OS X or Kindle platform.