A Note On How to Redact PDF Files

October 19, 2019

Redacting PDF files takes special consideration as it's a digital vector format that does not mean what you see is what you have.

Open the PDF in LibreOffice Draw. If the PDF has text, just change the text and the background color to be the same color, like black. Or, even if it is text, just use the rectangle tool to draw a black box over the text of whatever you want to redact.

Now, Export as PDF in LibreOffice.

Warning! Your PDF is not secure yet. For example, if there was text in there before, it's still in there even though it looks visually covered up.

Now, use Imagemagick's convert command to convert from pdf to pdf.

convert -density 300 in.pdf out.pdf

While it may seem like this does nothing, it actually does something important. convert always rasterizes the content, so your PDF will basically turn into images. Now, the PDF is digitally redacted. You'd get a similar effect, for example, if you printed the PDF and then scanned it back to PDF.

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