How to redact a PDF properly
Court filings, contracts and reports have repeatedly leaked “redacted” information because the black boxes were only drawn on top. Here is how to make sure what you hide is really gone.
Why a black box hides nothing
A PDF is made of layers of objects. Drawing a black rectangle — with a highlighter tool, a shape or an annotation — adds one more object above the text. Selecting the area and copying, or extracting the text with any program, still reveals the words underneath.
Changing the text color to black, or covering a scan with a shape, has the same problem.
What real redaction does
Real redaction removes the content under the marked area from the file. Redact PDF turns each page with marked areas into an image with the areas already painted, then rebuilds the document copying only what the pages use: the original text, fonts and images of those pages are not in the new file at all. Pages you do not mark stay as they were.
Finding everything
Search for each name, ID number or account number: every occurrence in the document is marked, regardless of accents or capital letters. The automatic searches add all email addresses, phone numbers and long numbers. Then go through every page, add areas by hand for signatures, photos and stamps, and remove any mark you do not want.
Before you send it
Open the result and try to search and copy the hidden text: it must not appear. Check the document properties too (author, title) — PDF metadata clears them — and remember that a file name like “Smith-dismissal.pdf” can say more than the document.