It only works with text based files. Basically, the 'redacted' parts were highlighted in black (think highlight tool in Adobe Acrobat) instead of being replaced with blacked out sections. As it was only highlighted in black in the redaction tool/PDF app - you could highlight all the text in the document, then copy paste it into notepad or similar, revealing the 'redacted' parts.
Many PDF's and doc's are images and the technique will not work for those.
Yep
>Many PDF's and doc's are images and the technique will not work for those.
To be clearer - PDF's can contain text or images, so it will work for PDF's where they contain text and the incorrect 'black highlights' were used to redact. Doc's containing handwritten notes will usually be PDF'd as an image where there's not much you can do (unless they added a redaction layer and left it in the PDF as a separate layer instead of flattening to one layer)
116