I'd suggest uploading the PDF to your store using the Files page in the Shopify admin (found under Settings > Files). Once you upload the file you will be able to copy the file's URL.
You can then add a menu item to your footer menu that links directly to the file URL using the "external link" option. Alternatively, you could add a menu item that links to a page in Shopify, and the content of that page could contain the URL for the PDF file.
See here for more details:
Hopefully this helps!