It’s been reported that yet again sensitive information has been posted on the web because people don’t understand the difference between what you see and what you get. A pdf document posted by the Civil Aviation Authority contained blacked out sections that were about airport security. However, the sensitive sections could be read quite easily: they just used black on black, or something similar.
Pdf documents contain all the text that was in the document from which they were produced; just because it’s not immediately visible (black on black, or a black block superimposed) it doesn’t mean that it’s disappeared. Just select the text to see it, or if necessary copy and paste into another application, or view the pdf file with a text editor.