A .acsm file is not an ebook and can not be converted. It is a file that will allow you to download the ebook from an Adobe content server (acsm => Adobe Content Server Message). Whether you will download the book as pdf or epub depends on what book format you elected to download.
If you look at the .acsm file with a text editor, you will see a line starting with <dc:format and ending with application/epub+zip</dc:format> or application/pdf</dc:format>depending on which format you elected to receive.
You can use
ADE for PC or Mac or the
ACSM Input plugin for calibre to download the ebook. You will need to have an Adobe ID to be able to download and open a encrypted epub or pdf. If you don't already have an Adobe ID, see
How to create your Adobe account.