How to redact a PDF properly (and why a black box often hides nothing)
A PDF is not a picture of a page. It is a list of instructions: "write these words here, draw this box there". A black box drawn on top just adds one more instruction, and the words are still in the file.
Why covering doesn't work
Text in a PDF stays text even when something is drawn over it. A black rectangle, a black highlight in Word, or a box from a free PDF editor changes only how the page looks. Anyone can:
- select the area and copy it into a note,
- press Ctrl+F (⌘+F on a Mac) and search for a word that should be hidden,
- open the file in another program that draws the text but not the box,
- or let a screen reader read the page aloud.
This has caught out courts, governments and big companies many times, because the redacted copy looks perfect on screen and on paper.
Ways that really remove it
Adobe Acrobat (paid)
Use the Redact tool rather than a drawn shape. You mark text or areas, then apply the redactions; until you apply them, nothing is removed. Acrobat can also remove hidden information such as metadata, comments and hidden layers in the same pass. Save as a new file.
LibreOffice Draw (free)
Open the PDF in LibreOffice Draw, use the Redact tool (or Auto-Redact to find words for you), draw over what you want gone, then use its option to export the redacted document as a PDF. The result is made from pictures of the pages, so the covered text is not there to copy.
Any computer: rebuild the PDF from images
Cover the words, then turn every page into a picture and make a new PDF from the pictures. On Linux or a Mac with Poppler installed, pdftoppm -r 200 -png covered.pdf page makes one PNG per page; any "images to PDF" option (or img2pdf page-*.png -o clean.pdf) puts them back together. The new file can't be searched or copied anywhere, which is the point, but it is larger and no longer accessible to screen readers.
Word and Google Docs
Delete the words and type [REDACTED] instead, then save or export a new PDF. Black highlighting is not redaction. In Word, File › Info › Check for Issues › Inspect Document finds comments, tracked changes and hidden text that could still contain the original.
Leftovers people forget
| Where | What can survive |
|---|---|
| Document properties | Author name, company, original title, editing software. |
| Comments and tracked changes | The deleted words, word for word, in the margin or the change history. |
| Bookmarks and headings | A section title that contains the hidden name. |
| File name | "letter-to-J-Smith-final.pdf". |
| Earlier versions | Some PDF editors append changes, so an older copy of the page stays in the file. Saving as a new file avoids most of this. |
| Images inside the PDF | A box over a scanned page may sit on top of the full scan; it needs the same treatment. |
The two-minute check before you send
- Open the exact file you will send, in a different PDF viewer from the one you edited in.
- Search (Ctrl+F) for one hidden word, a name or a number. It must not be found.
- Drag-select across a covered area, copy, and paste into a plain note. Nothing hidden should appear.
- Look at the document properties and remove anything personal.
Related
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Questions
Why can I still copy text under a black box in a PDF?
Because the box was drawn on top of the text instead of replacing it. The words are still in the file, so selecting, copying or searching finds them. Use a real redaction tool that removes them.
How can I redact a PDF for free?
LibreOffice Draw has Redact and Auto-Redact tools that export the pages as pictures. Or cover the words, convert each page to an image and make a new PDF from the images.
Is black highlighting in Word a redaction?
No. The text keeps its letters under the highlight and comes back when the highlight is removed or the text is copied. Delete the words instead and type a placeholder.
How do I check that a PDF is properly redacted?
Open the final file in a different viewer, search for a hidden word, and try selecting and copying the covered area. If nothing comes up, the text has gone.
Does printing to PDF remove the hidden text?
Not reliably. Many print-to-PDF options keep the text as text, so covered words can survive. Use a redaction tool or rebuild the PDF from page images.
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