a) a button click could open a new Filemaker window and its layout can use a large format interactive container to display the PDF
b) If you used insert file with the "store a reference" option enabled to insert the file. You should be able to just double click the container field to open the PDF in whatever application your computer is set to use as the "default application" for opening PDF's.
c) a script can use export field contents to export a copy of the file and open it. It's possible to export to the computer's temporary folder so that you don't leave extra copies of the PDF cluttering up your computer.