The NSA on How to Redact
Both the Microsoft Word document format (MS Word) and Adobe Portable Document (PDF) are complex, sophisticated computer data formats. They can contain many kinds of information such as text, graphics, tables, images, meta-data, and more all mixed together. The complexity makes them potential vehicles for exposing information unintentionally, especially when downgrading or sanitizing classified materials. Although the focus is on MS Word, the general guidance applies to other word processors and office tools, such as WordPerfect, PowerPoint, Excel, Star Office, etc.
This document does not address all the issues that can arise when distributing or downgrading original document formats such as MS Word or MS PowerPoint. Using original source formats, such as MS Word, for downgrading can entail exceptional risks; the lengthy and complicated procedures for mitigating such risks are outside the scope of this note.
EDITED TO ADD (2/1): The NSA page for the redaction document, and other “Security Configuration Guides,” is here.