A black box on a PDF: why the hidden text is still readable

Why a black box does not hide the text

A PDF file stacks several layers: a text layer (the characters, their font, their exact position on the page) and, possibly, graphic objects added on top (shapes, images, annotations). When you draw a black box with the “shape” or “drawing” tool of a PDF reader, you add a graphic object to that second layer. The text in the first layer does not move: it stays exactly where it was, with the same characters, in the same position.

The result: on screen, you no longer see the text. But for the computer, it is still there. Select all the content of the page (Ctrl+A or Cmd+A) and copy it into a text editor, and you get it back in full, black box or not. The search function of the PDF reader finds it just as easily. That is why people speak of a failed redaction: redacting originally means actually removing a passage, not covering it up visually.

The Manafort case: a redaction gone wrong

The case most often cited to illustrate this problem dates back to January 2019. A court filing in the Manafort case, in the United States, contained passages that were supposed to be hidden by black boxes. Journalists simply selected the text of the document and pasted it into an editor: the hidden passages appeared in plain text. The mistake did not come from a lack of sophisticated tools, but from the choice to cover the text visually instead of deleting it from the file.

The episode was widely shared because it showed, very concretely, that the problem affects everyone: government agencies, law firms, journalists, companies. If you are looking for a reliable end-to-end method, our guide on how to redact a PDF step by step details the procedure that avoids this kind of mistake.

How to check that a PDF is really redacted

Before sending or publishing a document, a few simple checks are enough to spot a failed redaction:

1. Select all the text (Ctrl+A or Cmd+A in the PDF reader), copy, then paste into a plain text editor (Notepad, TextEdit, Notes). If a hidden passage shows up in the paste, the redaction failed.
2. Search for a known word behind the box with the reader's search function (Ctrl+F or Cmd+F). If it is found, the text is still there.
3. Compare the file size before and after redaction, as a clue only: actually deleting text and images often changes the size, while a box added on top barely does, but recompression or a change of software can also make it vary. It is a weak clue: rely on the two tests above first.
4. Check the metadata and earlier versions kept inside the file, which may contain exactly the information you are trying to remove. Our article on how to remove a PDF's metadata explains what it reveals and how to clean it.

Common mistakes that leave text readable

Most failed redactions come from the same reflexes, all born of a confusion between “hiding visually” and “deleting”:

• Drawing a black box with the shape or drawing tool of a PDF reader.
• Using the highlighter set to black instead of a real redaction tool.
• Setting the text in white on a white background, or shrinking it to zero, thinking it becomes invisible for good.
• Pasting an opaque image over the text without touching the underlying text content.
• Adding annotations or comments in the belief that exporting or printing to PDF is enough to “flatten” them: the text stays underneath, and some readers let you move or delete the annotation, revealing what it hid. Real flattening means converting the page to an image, or using a redaction tool that deletes the text.

How to hide text reliably

To hide text in a PDF with no copy-and-paste risk, you must either actually delete the characters concerned, or turn the page into an image after covering them, so that no text layer exists at that spot anymore.

A real redaction tool does this job: the one in Adobe Acrobat Pro (“Redact”, on subscription), the one in Preview on a Mac (Tools → Redact) or the one in LibreOffice Draw (Tools → Redact, which turns the pages into images). Their drawing tools, on the other hand, only cover the text, and neither exporting nor printing to PDF deletes it. The step-by-step method is in redacting a PDF: the reliable method, step by step and, on a Mac, in redacting a PDF on a Mac. In every case, reopen the final file and run the tests above again.

Online tools (Smallpdf, iLovePDF, UPDF…) sometimes offer redaction or text-removal features. Before uploading a confidential document, check their privacy policy: the file is sent to their servers to be processed, which can be a problem for documents covered by professional secrecy or for sensitive personal data.

AnonymFiles is another option, designed precisely against this trap: redacted text is actually deleted from the output PDF, which keeps its layout, and the black block carrying the label hides nothing; metadata, annotations, forms and links are removed along the way. The app detects the information to hide (people, organizations, addresses, emails, phone numbers, IBANs), replaces it with stable labels such as [PERSONNE_1], also allows manual redaction, and shows the original and the copy side by side so you can approve each document before export. It processes an entire folder, with no account, no online license check, no telemetry and no cloud chat, and works offline once installed; it is coming soon for Apple silicon Macs and Windows. Scanned PDFs and images are read by local OCR, and the pixels of the detected areas are erased in the image. Whatever tool you choose, keep the habit of reviewing the final result: no automatic tool is 100% reliable, and for a document meant to circulate, the guide anonymize a PDF for free without uploading it covers the whole process.