How do I save the note PDF somewhere of my own, or open it in Preview?

With a note preview open, the top of the panel has buttons to open it in your Mac's normal PDF viewer (e.g. Preview) and to "Save a copy". Save a copy opens the standard macOS save dialog (defaulting to Downloads) so you can keep a plain PDF wherever you like. Both give you an ordinary, readable PDF — handy for emailing or filing. The app's own working copy stays encrypted; these buttons hand you a decrypted copy on purpose.