The PDF, titled "SUCCESSOR SSBN - SAFETY REGULATORS' ADVICE ON THE SELECTION OF THE PROPULSION PLANT IN SUPPORT OF THE FUTURE DETERRENT REVIEW NOTE", was published on the parliamentary website following requests under the Freedom of Information Act. Although sections were supposed to be protected through redaction, the method used to do so was very sloppy, according to according to The Telegraph.
The UK government merely turned the text background to black. The words were made unreadable, sure, but only in a given PDF reader. The text was still there, meaning that a user could simply copy and paste the blacked-out sections to another document. After media outlets reported the error, the issue was rectified by using an image of the document in question.
This was a very basic error to make given that anyone with an elementary knowledge of computers knows how to read the "redacted" content. Properly redacting content is not difficult, but if you don't know how, Adobe offers a simple guide on how to do so: Rick's Guide to using Redaction in Acrobat X Pro.
Thankfully the document in question was marked Restricted before it became Unclassified. The five levels are as follows: Unclassified, Restricted, Confidential, Secret, and Top Secret. Still, we hope this is a lesson for anyone who needs to redact information in a given document.