Can I make PDF the default for 'print to file'

You can install the cups-pdf package, add a new printer of the type "Generic CUPS-PDF Printer", and then make that the default printer.

I don't use this myself, so I am not sure if the quality of the PDFs that creates is different (for better or for worse) than what the "save to file" feature provides though. (Maybe you can comment here on your experiences with it?)

Here's how I made PDF the default:

Make Print-to-File default to PDF in Firefox.

First, open Firefox, then:

1. about:config
2. print.print_to_filename

3. /home/user/Desktop/mozilla.pdf

   If the above instructions do not work, continue with the following:

4. Print something from an installed printer using Firefox (entry may not show otherwise)
5. about:config
6. Type "print_to_filename"
7. Identify the entry that has the exact name of the printer attached to your computer (e.g., print.printer_Canon-xxxx.print_to_filename)
8. Change the Value to: /home/user/Desktop/mozilla.pdf