everyday mac tools

Redacting a PDF in Preview on a Mac, and what Preview actually does

· 6 min read

Preview can redact a PDF. Since macOS 11 the Tools menu has a Redact item, and unlike a black rectangle from the shapes tool, it removes the text and image content under the mark from the file when you save. For a name on a page or a paragraph in a letter, it is the right tool and it is already installed.

What it does not do is check its own work, strip the rest of the document, or find every instance of a word for you. This post walks through using it and then through the gaps, so you know which checks to run afterwards.

Duplicate the file first

Preview saves edits into the file you opened, automatically, and redaction is designed to be irreversible. Before you draw anything:

  1. Open the PDF in Preview.
  2. File, Duplicate (Shift-Command-S). A copy opens in a new window.
  3. Save the copy with a name that says what it is, such as “letter, redacted”, and close the original.

Work on the copy. macOS keeps earlier versions of documents you edit (File, Revert To), which is useful if you over-redact, and is also a reminder that your Mac may hold the unredacted content in its own version store. That store never leaves the machine, but do not treat the redacted file’s history as private on a shared Mac.

Redact text

  1. Tools, Redact. Preview shows a warning that the content will be permanently removed when the document is saved. Accept it. The Redact tool also appears in the Markup toolbar (View, Show Markup Toolbar) as a marker icon.
  2. Drag across the words to remove, as you would to select text. A black box appears over the selection.
  3. Continue through the page and the document. Each mark is a separate action, so you can undo the most recent one with Command-Z until you save.
  4. When you have finished, File, Save (Command-S), or File, Export to write it under another name.

On save, Preview removes the glyphs beneath each box from the page content and leaves the box in its place. The words are not covered; they are gone. Select All and Copy afterwards produces the text around the box and nothing from inside it.

Redact part of an image or a scan

For a scanned page, or a photograph inside a document, the same tool works by area rather than by text.

  1. Tools, Redact.
  2. Drag a rectangle over the region.
  3. Save.

Preview changes the pixels in that region of the image, so the original picture data is no longer in the file. Zoom in on the result to confirm the box covers the full area including any fringe, and read the section below on text layers, because a scan that was made searchable has a second copy of the words that you cannot see. Redacting photos and scans inside a PDF covers the image case in detail.

What Preview does not do

This is the part that matters if the document is going anywhere consequential.

It does not verify. Preview draws, saves, and stops. It does not re-open the output and confirm the strings are absent. That is your job, and How to check whether a PDF was really redacted is the routine: Command-A and Command-C into TextEdit, Command-F for the removed words, mdls in Terminal, the Inspector.

It does not strip the rest of the file. Redaction removes content from the page. The document’s title, author and keywords fields, the XMP block, bookmarks, attachments, hidden layers and any thumbnail images the producer embedded are outside the page, and Preview’s Redact tool does not claim to touch them. Check the Inspector (Tools, Show Inspector) and mdls before you send, and if any of those hold the words you removed, follow Remove metadata from a PDF on a Mac.

It does not search. There is no way to tell Preview “every occurrence of this surname”. You find each one with Command-F and mark it by hand. On a letter that is fine. On a three-hundred-page production it is where names get missed.

It does not review. Marks apply as you draw them and are committed on save. There is no list of pending marks to read through and approve before anything is destroyed, and no way to re-run the same set of marks on the same file later and get an identical result.

It does not log. If you need a record of what was redacted and why, for a privilege log or a records request, Preview gives you nothing to build it from; you keep that list yourself.

When Preview is enough, and when it is not

Preview is enough when: the document is short, you can see every instance of what needs removing, nothing outside the page carries the sensitive words, and you will run the checks above on the output.

It is not enough when the document set is large, when the same names recur across many files, when you must show your work to someone else, or when a single miss has a real cost. For that work, Basalt is built around the same idea taken all the way: text excised glyph by glyph with the survivors re-emitted in place, images destroyed and re-encoded, metadata and XMP and attachments and layers and stale thumbnails stripped, every mark reviewed before it is applied, and a verifier that re-opens the export with an independent parser and refuses to write the file if any removed string is still present. Each export gets a signed certificate you can check without the app. Same category of tool as Preview’s Redact; different amount of proof.

A short checklist

  1. Duplicate before you start.
  2. Find every instance with Command-F; do not trust your eyes on a long document.
  3. Redact with Tools, Redact, never with a shape.
  4. Save, then reopen the saved file, not the window you were working in.
  5. Command-A, Command-C, paste into TextEdit, search.
  6. Inspector and mdls for the fields.
  7. Rename the file before sending.

Questions

I drew black rectangles with the shapes tool before I knew about Redact. Are those safe? No. Shapes are annotations drawn over intact text. Delete them, then use Tools, Redact on the same areas. Why a black box on a PDF does not redact anything shows how quickly the shape version fails.

Can I redact in Quick Look’s Markup instead? The Markup extension in Quick Look and Mail gives you shapes and highlights, which cover rather than remove. Use Preview’s Tools menu for the real one.

Does redacting in Preview change the rest of the page? The visible layout stays as it was. The text around a removed passage is re-written so the line still reads, which is why copied text can show a gap or unusual spacing where the words were.

Why is Redact missing from my Tools menu? It arrived with macOS 11. On anything older, Preview only has shapes, and shapes do not redact.