Redact a PDF

Is a black box over text enough to redact a PDF?

No. A black rectangle drawn in a PDF editor only sits on top of the text, and the words underneath can still be copied, searched or revealed by deleting the shape. Court filings and government reports have leaked names and figures exactly this way. Highlighting in black, changing the font colour to match the background and covering text with a comment all have the same flaw.

Real redaction removes the content itself. This tool does that by replacing each marked page with a flat image in which the covered areas are solid black pixels. There is no text layer on that page left to copy, and the original page objects are not kept anywhere in the new file, not even as unreferenced leftovers that a forensic tool could dig out.

What does this redaction actually remove from the file?

Everything on a marked page is removed and replaced by a single JPEG image: text, vector drawings, embedded images, annotations, links and form fields. The page is rendered with pdf.js at 200 DPI, each box is painted opaque black with one extra pixel on every side so no anti-aliased edge of a letter shows, and the picture is saved at the original page size.

The output is a brand-new document, not an edited copy. In the PDF format described by ISO 32000-2, a saved file can hold objects that no page refers to any more, so editing in place risks leaving the old content behind. Here only the needed pages are copied into an empty file. Marked pages are also emptied in the source first, so an untouched page that links to a redacted one cannot drag its original content along.

What should be redacted before sharing a document?

Redact any detail the recipient does not need for the purpose at hand, which usually means identifiers, contact details and figures. Before sending a document to a landlord, employer, court, insurer or journalist, read it once with that single question in mind and mark everything that fails the test. The most common candidates are:

• Account, card and policy numbers on statements
• National ID, passport and tax numbers
• Home addresses, phone numbers and signatures
• Names of children, patients or witnesses
• Salary figures and pricing in shared contracts

How do I make sure nothing is missed?

Use Find text for anything that repeats, then review every page by eye, because a search can miss words split across lines or written in unusual fonts. Try variations such as an account number with and without spaces, or a first name on its own. Draw boxes slightly larger than the text so the tops of tall letters and the tails of others don't show.

Check headers, footers, stamps and signatures, and remember that the same fact can appear in a chart label, a table or a photo. After downloading, open the result and try selecting or searching for the redacted words to confirm nothing remains. Keep the unredacted original somewhere private, because redaction can't be undone in the new file.

What are the limits of image-based redaction?

The main limit is that redacted pages become images, so they are larger than text pages, can't be edited as text and are not searchable until OCR is run. Long documents with boxes on hundreds of pages take a while on phones because each page is rendered one at a time. For a smaller file afterwards use Compress PDF, and to add a password before sending use Protect PDF.

Pages left untouched keep their text, links and quality exactly as they were. If a document needs only a few words hidden on a long text page, that whole page still becomes an image, so consider whether sending only the relevant pages with Split PDF suits the recipient better. Files up to 100 MB are supported, and a password-protected PDF is saved without its password once redacted.