In this guide

Do not use a black box
See why: select under the box
The number is still there
Use Tools, Redact instead
Check the redaction
Test the saved copy
Only the label is left
If you do not see this option
If this does not work
Good to know
Sources

Short answer

In Preview, choose Tools, Redact, drag over the text and save. Do not just draw a black rectangle with Markup: when we tried it, the number under the box could still be selected, copied and pasted.

Device: Mac
Shown on: macOS 27.0.1 (MacBook Neo)
App: Preview
Path: Preview › Tools › Redact
Real screenshots: 7 of 7 steps
Checked: October 2026
Sources: 1 official, linked below

Step by step

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Do not use a black box
A filled rectangle from the Markup toolbar covers the number on screen, but the text is still in the file.
Done
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See why: select under the box
In a saved copy, we dragged across the line, including the black box. The text under it was selected.
Done
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The number is still there
Pasted into TextEdit, the copy read "Test account 1234 5678".
Done
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Use Tools, Redact instead
Choose Tools, Redact, then drag over the text. Preview warns that redacted content is permanently removed when the document is closed. Click OK.
Done
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Check the redaction
The redacted text shows as a hatched box. Save the file, or save a copy and keep the original somewhere safe.
Done
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Test the saved copy
Open the saved copy and select the same line again.
Done
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Only the label is left
Pasted into TextEdit, the line read just "Test account", followed by the next line of the file. The number was gone.
Done

If you do not see this option

No Redact in the Tools menu? Open the PDF in Preview, not in Safari or another viewer. Redact works on PDFs; for an image, crop the part out instead.
Looking in the Markup toolbar? Apple's Preview guide also describes a redaction tool in the Markup toolbar, and says to redact a duplicate so the original stays safe. Source: Annotate a PDF in Preview on Mac

If this does not work

Still afraid it leaked? Do what we did: select the line in the saved copy, copy and paste it into TextEdit. Only what you kept should paste.
Already sent a boxed PDF? Assume the text can be read. Redact it properly and send it again, and change anything sensitive it contained.