I thought the Save button in the built-in PDF viewer worked the same way as Save Page As... (Ctrl+S). I don't suppose there is a policy to lock Save Page As (or the context menu Save [object] As... entries) to a specified downloads directory?

(I'm going to flag this thread to be moved over to Firefox for Enterprise since there should be more policy gurus there.)

There's a bug being worked on to remove Save dialogs:

https://bugzilla.mozilla.org/show_bug.cgi?id=1708289

but we currently don't have a way to do this.

As a work around, you can force the download directory to something like /dev/null and that should work (in theory), although I haven't tested with pdf.js specifically.

Firefox 124 will allow blocking file dialogs with the AllowFileSelectionDialogs enterprise policy.