I do not know if it's what you were looking for, but it sounds to me something like this.
This is for linux (ubuntu) but maybe it also works on mac:
If the file is a pdf called 'book.pdf' and is located in your downloads, then
import subprocess
path='/home/user/Downloads/book.pdf'
subprocess.call(['evince', path])
where evince is the program that open pdfs in ubuntu