If a document is annotated, Find functions will continue to work.  Note that in some cases it might be difficult or impossible to see highlighted text behind an annotation, but it will highlight and be considered in next/previous functions.  In the example below, you will see the text document management is highlighted behind the received stamp.

For users with Modify Annotation permissions, text behind redactions will also be highlighted.  You would still need to move or remove the redaction to see the text.  Users without annotation permissions would not be able to search for text in the document to begin with, so security is not in question in this scenario.  Any pages with annotations for a user without the required Modify Annotation would not be text searchable.  Pages without annotations would remain searchable.