These Preferences are targeted for handling PDF files.
Open In Acrobat
Always use the System Default Application
By default the macOS always uses the latest install of Acrobat or Preview on a Mac as its Default app to handle PDF files. \
Always use a specific install
Selecting this radio button allows you to FORCE a specific application to always be used to open the PDF files. This is like Right-Clicking on your PDF file in the Finder and selecting an application to open it. Selecting this Preference causes no IDML conversion to be attempted, instead, if this fails, you will receive an error message from the application you selected to open the PDF file.