How to redact a PDF properly on a Mac

Why a black box is not a redaction

A PDF is built in layers. The text you read is stored as actual characters, and anything you draw, such as a shape, an annotation or a highlight, is usually added as a separate object on top. When you draw a black rectangle over a name, the name is still there underneath. It is hidden from your eyes, not removed from the file.

That hidden text can come back in several ways. Someone can select across the box and copy the words into another document. A search for the name will still find it. Moving or deleting the annotation in another PDF app reveals it. Screen readers and text extraction tools read it out. Court filings and official documents have exposed text this way when people assumed a black box was enough.

Setting the text colour to black, or using a black highlighter, has the same problem. The words are still characters in the file; they just happen to be hard to see.

Covering versus removing

It helps to think of two different things that both look like redaction:

• Covering puts something opaque on top of the content. The page looks redacted, but the original text, and sometimes images, remain in the file.
• Removing deletes the content from the file itself, then marks the spot, usually with a solid box. Nothing remains to copy, search or uncover.
• A common way to remove is to turn the affected page into an image with the boxes painted in, so there is no text layer left beneath them. Running OCR afterwards makes the rest of the page searchable again without bringing back the removed words.

Hidden places sensitive information can survive

Text on the page is not the only place information hides. Before you share a redacted PDF, think about:

• Metadata: the document's title, author, subject, keywords and the app that created it can include names or case details.
• Bookmarks and outlines, which may repeat headings you redacted on the page.
• Comments, notes and form field values that sit outside the main text.
• Hidden layers in PDFs exported from design or drawing apps.
• Attached files embedded inside the PDF.
• Earlier versions: some editing workflows append changes to a PDF rather than rewriting it, so older content can remain inside the file.

A safe redaction workflow

Whatever tool you use, the same steps apply:

• Work on a copy. Keep the unredacted original somewhere safe and never send it by mistake.
• Find everything first. Search the document for each name, number and term you need to remove, including variations and partial matches.
• Use a tool that removes text, not one that only draws shapes.
• Burn in or flatten the result so nothing sits underneath the boxes.
• Remove metadata and check bookmarks, comments and attachments.
• Verify the final file with the tests below before it leaves your computer.
• Keep the redacted file on your own computer while you work. Uploading a sensitive document to a website for redaction defeats the purpose.

Redacting a PDF with FileDial

FileDial includes several redaction tools that run on your Mac, so the document is never uploaded. Select the file in Finder, hold the right Option key and choose from the dial. Each tool saves a new file beside the original, which stays unchanged.

Sign & Lock › Redact text… finds the words you specify and turns matching pages into images with black boxes, then runs OCR again so the rest of the page stays searchable. Sign & Lock › Find and redact personal info… looks for personal details and produces a redacted copy, along with a .txt file listing what kinds of information were removed.

If you have already drawn black boxes in another app, Sign & Lock › Check redactions writes a .txt report of any text still hidden under them, and Sign & Lock › Burn in redactions turns pages with black boxes into pictures and re-runs OCR, so the hidden text is really gone. For photos and scanned images, Privacy › Redact by hand… lets you place solid boxes yourself; they are burned in and metadata is removed.

Read more on the redact a PDF on Mac page. For faces and details in photos, see blur faces and redact photos.

What about Preview?

Recent versions of Preview on macOS include a redaction tool in addition to the usual shape and highlight tools. Use that dedicated redaction tool rather than drawing a black rectangle with the shape tools, which only covers the text. Whichever app you use, do not assume the result is clean: run the checks below on the saved file.

How to check that a redaction worked

Always test the final file, ideally after reopening it, rather than trusting what you see on screen.

• Search test: open the redacted PDF and search for each word or number you removed. If the search finds it, the redaction failed.
• Copy and paste test: select all the text on a redacted page, copy it and paste it into a plain text editor such as TextEdit. Look for the removed words.
• Selection test: try dragging across a black box. If you can highlight text inside it, something is still there.
• Metadata check: in Preview, open the inspector to see document details such as title and author, and make sure they do not reveal what you removed.
• Automated check: run a tool such as FileDial's Sign & Lock › Check redactions, which reports text still hidden under black boxes.

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