Redact a PDF

Redact a PDF properly

Type the words to remove, or pick a pattern like every email address, and each match is blacked out. Then the pages are re-rendered, so the text underneath the boxes is gone from the file rather than hidden behind a rectangle.

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Or match a pattern

This removes the text, it does not cover it. Every page is re-rendered after the boxes are drawn, so the words are not in the output file at all — which is the whole point, and also why the document stops being searchable. Keep the original.

How to redact a pdf properly

  1. 1

    Drop in the PDF

    The text layer is read on your device. Nothing is uploaded at any point — which matters more here than anywhere else on this site.

  2. 2

    Say what to remove

    One term per line, or comma-separated. Add patterns to catch every email address, phone number, card number, national insurance number or IBAN without typing them out.

  3. 3

    Redact

    Each match is located, boxed in black, and the page re-rendered with the box burned in.

  4. 4

    Check before you send

    The result reports how many matches it found. Open it and search for the terms — they should return nothing.

Why a black rectangle is not redaction

Drawing a black box over a name in most editors leaves the name in the file, underneath. Selecting the area and copying it, or running any extraction tool, brings it straight back. This is the single most common way confidential documents leak. Here the page is re-rendered after the box is drawn, so the characters are not in the output at all.

The cost is the text layer

Removing text by re-rendering means every page becomes an image. The document is no longer searchable or selectable, and screen readers cannot read it. That is a real cost, and it is the price of the guarantee — keep the original somewhere safe.

Matches are found across fragments

PDFs split words across text objects for kerning, so a naive search misses half of them. The page text is reassembled into one string with a map back to each fragment, so a term split between two objects is still found and each part boxed.

Questions

Not from the output file — the characters are not in it. What redaction cannot do is protect against inference: if a name is redacted in one place and not another, or the black box is exactly as wide as the word, the content can still be guessed.

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