It says:

Default styles are provided to match the styles in Qt Quick Controls. File selectors are used to load the PDF style corresponding to the Controls style in use. Custom styles are possible, using different file selectors.

So custom styles are possible using file selectors... But unless I am missing something, using file selectors would mean the user has to modify Qt PDF module itself, add a custom style there and pick that using file selectors.

If so, that almost sounds like custom styles are not possible unless you modify Qt PDF.