Mashable broke the story on a certain Joost PDF document which contains a lot more than it initially seems. Found in the wild, on a server of one of Joost’s employees, the file is a PDF presentation of some of Joost’s technicalities. But simply selecting the text, copying and pasting it to notepad reveals a whole lot more data. Yes, some of it is definitely not meant to be seen by the public; it includes a number of Joost’s upcoming deals with media content providers.

Let’s quickly explain how can this happen. PDFs are usually used by the government precisely because they’re quite safe: they contain very little metadata, unlike Word documents which can (depending on the version) contain a lot of hidden stuff. My (educated) guess is that in this case, someone created one Powerpoint presentation. Then opened the same document, and created new boxes on top of old ones, which thus became invisible. And after that, (ironically) probably to be on the safe side, they converted the entire thing to PDF thinking that the old data is gone. But, it’s not - it’s simply not immediately visible.

To illustrate how this works, I’ve created a simple PDF document with hidden text underneath the image, in the same way: by creating a Powerpoint presentation first, and then printing it to PDF format. You can check it out here; to see the hidden text, select all, copy and paste to notepad.

The moral of the story is: never, ever convert sensitive data from Powerpoint presentation to PDF. It might get you into a lot of trouble.