When it comes to breaking into protected information, the NSA is the place to go. They know the tricks. They can also tell you how to keep your information secure. In a nutshell: Don't redact, sanitize.Download their pdf...
Redacting with Confidence: How to Safely Publish Sanitized Reports Converted From Word to PDF - Information Assurance Directorate, National Security Agency
I couldn't find anything redacted in it, but I am still suspicious about the second page.
