How to redact a PDF: the reliable method, step by step

Redacting a PDF: the exact definition

Redacting a PDF means permanently hiding a specific passage: a name, an account number, a clause, a signature. Contrary to what the word suggests, the visible part (the black box) is only the consequence of the operation: what matters is that the underlying text is actually deleted, not just covered. People also talk about “blacking out” or “striking out” text, but the expected result is the same: the information cannot be recovered, whether by selecting the text, zooming into the image or inspecting the file.

It is a more targeted operation than anonymizing a PDF, which processes the whole document and replaces identifiers with consistent labels rather than simply hiding them. Redaction is enough when you want to remove one or two specific items before sharing a document; anonymization is needed as soon as the document contains many identifiers that must be handled consistently.

In practice, “redacting” covers several gestures: censoring a contract clause, striking out a social security number, blacking out a signature or an ID photo. The principle is the same in every case: the final result must keep no recoverable trace of the original content, neither in the text, nor in the image, nor in the file's metadata. To redact a scan or an identity document, where the text is only an image, the method changes.

Redacting a PDF with Adobe Acrobat

Adobe Acrobat Pro offers the most complete tool for redacting a PDF, with built-in text search. In the English interface, it is called “Redact”.

1. Open All tools → Redact a PDF (Tools → Redact in earlier versions).
2. Choose “Find and redact text” to locate a word or pattern (name, email, number) throughout the document, or mark an area manually with the selection tool.
3. Check the list of marked occurrences before continuing: this is the time to fix anything you missed.
4. Click “Apply”. This is the action that actually deletes the marked text; marking alone is not enough.
5. In the next window, turn on “Sanitize document” to also remove metadata and hidden information, then save: the deletion only becomes final when you save.

The drawback is the price: this tool is reserved for paid Acrobat Pro subscriptions.

Redacting a PDF for free with Preview (Mac)

On a Mac, Preview lets you redact a PDF without paying or installing anything, but without automatic detection: on a copy of the file, choose Tools → Redact, select each passage, then save and close the document. Do not use the rectangle tool from the markup toolbar: even exported or printed to PDF, it leaves the text selectable underneath. The step-by-step guide, and the fallback if the tool is missing in your version, are in redacting a PDF on a Mac: Preview, Acrobat and alternatives; how this trap works, made famous by the Manafort case, is explained in black box on a PDF: why the text stays readable.

Redacting a PDF with LibreOffice or online

LibreOffice Draw has a built-in redaction feature: open the PDF in Draw, choose Tools → Redact, place the areas to hide, then use the redacted export offered by the redaction toolbar, which produces a PDF whose pages are images. Drawing simple black shapes and then exporting to PDF is not enough: the text stays underneath. It is free and the file stays on your computer, but the resulting PDF is no longer searchable, and nothing is detected automatically. Check the result afterwards with a copy and paste.

Online tools such as Smallpdf, iLovePDF or UPDF also offer redaction features, handy for a one-off document with no sensitive data. The trade-off to keep in mind: your file is sent to a third-party server, which adds an intermediary to the chain as soon as the document contains personal or confidential information. We compare the two approaches in anonymizing a PDF online or offline: which to choose?

Checking that a redaction really works

Whatever method you use, one single check tells you whether the redaction worked: open the final file, select all the text on the page concerned (Cmd+A or Ctrl+A), copy it, then paste it into a plain text editor. If the passage that should be hidden appears in the pasted text, the job is not finished, however the black box looks on screen. Additional checks (searching for a hidden word, metadata) are detailed in how to check that a PDF is really redacted.

On a case file of several documents or several dozen pages, repeating this check page by page takes time. Dedicated software such as AnonymFiles lets you redact by hand on a copy while relying on automatic detection for the rest of the document, then compare original and copy side by side and approve each document before export. In the PDF it produces, the redacted text is actually deleted, not just covered. The application lets you redact an entire case file offline once installed, with no account, no online license check, no telemetry and no cloud chat. Scanned PDFs are read by local OCR and redacted in the image itself, and the copy-and-paste test is still recommended on the final file. AnonymFiles is coming soon for Apple silicon Macs and Windows; for a single document, Preview or LibreOffice work very well if you take the time to check the result.