Hi there!
Before you export the annotations, you can loop over them and set them as readonly.
const { annotManager } = instance;
const list = annotManager.getAnnotationsList();
list.forEach(item => {
item.ReadOnly = true
});
const xfdf = await annotManager.exportAnnotations();
...
Keep in mind that a lot of PDF viewers do not honour the ReadOnly flag, so the document may still be editable in some viewers.
Thanks!
Logan