The UK Ministry of Defence has been caught out again by a schoolboy error not knowing how to properly redact a PDF. A 22 page PDF document called Air Defence And Air Traffic Systems Radar Transportation Study Part 2 was published on a parliamentary website. The act of obscuring the sensitive information is known as redaction, and it needs to be done properly if you want to keep something secret. For instance, simply putting black text on a black background does not stop people from cutting-and-pasting the contents.”]
Source: https://grahamcluley.com/how-redact-pdf-air-defence-radar-secrets-spilled/