But there is a lot of data which is in principle visible -- it is rendered as part of the document's visible image -- which in fact not visible to the human reviewer.
A human reviewer cannot see and redact all this quasi-visible information. For privacy, it is essential to eliminate such text, together with the metadata, and only the human-readable form of the document -- after suitable redaction, of course.- Text with the same color as its background: E.g, white on white
- Super-small text: 1 pixel high, effectively invisible
- Super-wide margins: The text is pushed off the edge
- Folded sections
- Layered sections
See also this excellent White Paper: "The Risks of Metadata and Hidden Information."