If you're an organisation that is making public an internal document, you best make sure that you have deleted or blacked out any personal, confidential or actionable information.
The act of obscuring the sensitive information is known as "redaction", and - for obvious reasons - needs to be done properly if you care about privacy and avoiding a potentially damaging data leak.
In the old days - before PDFs and Word documents - you might have redacted a document with a thick black marker pen, ensuring that anyone who made a photocopy of the document wouldn't be able to see the censored words. Things are different with electronic media, of course.
Unfortunately, time and time again we've seen sloppy security procedures make it far too easy for unauthorised parties to view information in electronic documents that should have been properly redacted.
The last example which has made numerous newspaper headlines, involves the British Ministry of Defence, which was found to have published a PDF document online, unintentionally revealing information about nuclear submarine security.
The PDF, entitled "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. However, although sections were supposed to be protected through redaction - it was possible to copy-and-paste the blacked-out text straight out of it.
Quack quack oops!
As the Daily Star explained:
The bunglers turned the text background black - making the words unreadable - but crucially left them in place. That meant anyone wanting to read the censored sections just had to copy the text.
This was a real school-boy error to make - as anyone with even an elementary knowledge of computers would know how to read the "redacted" content.
If you want to learn how to properly redact Adobe PDF files, here's a great guide describing how to do it with Acrobat X Pro.
Good luck, and remember that simply marking text will not actually remove it from your sensitive PDFs. You also have to apply redactions!