If you want long term access make sure you register your copy of ADE(Digital Editions) with an explicit Adobe ID BEFORE you open the .acsm file in ADE.
If you open the .acsm file in digital editions, it will create a (DRM) file, either .epub or .pdf.
It is already decided which you will get in the .acsm file; you can't choose.
(Sometimes there is a choice before you download the .acsm file; for example our library usually gives the choice)
Once you have the file you cannot convert it because of the DRM.
Even if you have a DRM .pdf you still need ADE or similar to open it, again because of the DRM.