Before emailing a PDF document like a bank statement or a legal contract, you must ensure that sensitive information—such as account numbers, passwords, or personal addresses—is securely obscured. Many users attempt to do this by simply drawing a black rectangle over the text.
This is a dangerous mistake. Drawing a shape over text does not delete the underlying text; it only hides it visually. Anyone who receives the PDF can simply copy and paste the text out from underneath the black box, or use a PDF editor to delete the shape entirely.
To safely obscure text, you must use proper Redaction. macOS includes a true redaction tool built directly into the default Preview app, which permanently destroys the underlying text data.
How to Safely Redact Text in Preview
The Preview redaction tool works by replacing the selected text with a black bar and simultaneously wiping the actual text data from the file’s code.
- Open your PDF document using the built-in Preview application on your Mac.
- Look at the top toolbar and click the Show Markup Toolbar button. (It looks like a small pen tip enclosed in a circle, located near the search bar).
- A second toolbar will appear below the main one. Click the Redact button. (It looks like a square with a solid black bar inside it).
- A warning message will immediately pop up on your screen. It warns you that redaction will permanently remove the content. Click OK.
- Your mouse cursor will turn into a crosshair. Click and drag your cursor over the sensitive text you wish to hide.
- As soon as you release the mouse button, the text will be covered by a black bar adorned with white “X” marks.
How Redaction Handles Images
The Redact tool is not just for text; it also permanently removes visual data. If you need to obscure a face in a photograph or a signature at the bottom of a document, the process is slightly different.
- With the Redact tool still selected, click and drag a large rectangle over the image or signature.
- Release the mouse button. The selected area will be replaced by a solid black box.
Just like with text, Preview does not simply draw a black shape over the image. It mathematically crops the image file embedded in the PDF and replaces those specific pixels with black, making it impossible for anyone to reverse the process using Photoshop or PDF editing software.
Saving and Finalising the Redaction
When you use the redaction tool, the changes remain editable only while the document is currently open. If you accidentally redact the wrong paragraph, you can press Cmd + Z to undo it.
However, the redaction becomes permanent the moment you close the document.
- Click File in the top menu bar.
- Click Save.
- Close the Preview window.
The text and image data beneath the black bars are now permanently destroyed. You can safely email the PDF knowing that the sensitive information cannot be recovered by the recipient.