@stonegatetraining we have now implemented a new feature called Global Scripts. This effectively allows you to run custom Javascript inside your account, thus making it useful for these kinds of default preferences.
Global Scripts are part of the advanced customisation tools so they will require a Power User Subscription. If you have an active subscription I can add this script to your account.
For anyone looking to implement this on their own you can add the following single line statement to the scripts section.
$("#chkattachPDF").attr("checked", true);