TL;DR
Drawing black boxes over text doesn't actually remove it from the PDF. Use Preview's Redact tool to permanently delete sensitive content.
How to do it
- Open the PDF in Preview (double-click usually does it).
- Click the Markup toolbar → select Redact (black rectangle icon).
- Highlight the text or area you want to redact.
- File → Export as PDF (or Cmd+S) — only now is the content actually removed from the file structure.
- Done: Redacted sections are permanently deleted, not just covered up.
Why this matters
If you just draw a black rectangle over text, the underlying data stays in the PDF file and can be extracted with simple tools. The Redact tool removes content from the file structure itself.
Good to know
- Metadata (author, creation date) isn't automatically removed — clean that up separately if needed.
- For complex PDFs (form fields, embedded objects), double-check with a professional tool.
- Especially important for lawyers, government agencies, and anyone handling confidential documents.