How do I do it so it's offline?
The FS9 kneeboard is just a browser it's no different from the browser you are reading this message in except it's smaller and does not permit pop-ups.
If the URL is on one's harddrive:
. . . as shown here or online on the internet, it will still be viewable in the kneeboard.
I use online and offline resources while flying FS9 but avoid opening (as much as possible) *.PDF files as they gobble up too much of my computer resources.
bill