By default the open PDF is handled by the user’s browser. I’m sure you don’t want to tell them to change settings, so here are the options.
Option #1: Put all your PDF’s in a compressed zip folder on the server, or whereever they’re located. Then link the button to that location. The user’s browser will not try to open. However, if is at their office, it’s likely the download will be block by their proxy or admin.
Option #2: You will need an additional html reference inserted into custom code for that file. Add it at the end of the a href reference. Here’s another thread about this as well:
I would suggest using the custom code first, then adding a zip file if you need to.