Redact a PDF permanently: remove the text, not just cover it
· 6 min read
Why a black box drawn over a PDF hides nothing, how to remove text, pictures and comments for good in your browser, and how to check that they are gone.
Why a black box hides nothing
A PDF page is a list of drawing instructions: put this text here, this picture there, this rectangle on top. When you draw a black rectangle over a name in an ordinary editor, you add one more instruction. The name is still in the file, under the rectangle. Anyone can select the text through the box and paste it elsewhere, search for it, open the file in a tool that lists the text, or simply delete the rectangle. The same goes for a highlighter set to black, a sticker picture, and a comment that covers the words. Covering is not removing, and the difference has embarrassed courts, companies and governments in public.
Redaction is the removal: the text, the part of the picture, the drawing and the comment under the box are taken out of the file, and only then is the box painted. What is left to find is nothing, because nothing is there.
Step by step
- Open Redact PDF and add the file. It opens in the editor with the redaction tool selected and the search set to find email addresses. The file stays in your browser; a document you are redacting is the last thing you want to upload.
- Draw a box over anything that must go: a name, a signature, an address block, a photograph. Drag the corners to fit. Each box is a mark for now; nothing is removed until you save, so you can adjust and undo freely.
- For things that repeat, use the search. Type a word, or pick a pattern (email addresses, phone numbers, card numbers), and choose Redact all: every match on every page gets a box at once. Run it again for the next word.
- Choose Save PDF. The engine removes every glyph a box touches, the underlines and highlights of those glyphs, every drawing mostly inside a box, every picture a box covers whole, and every comment, note or link a box touches, then paints the boxes. Pictures only partly under a box are rewritten with those pixels black.
- Download the result and check it, as the next section describes. Keep the original somewhere safe and send only the redacted copy.
How to check that it is really gone
- Open the redacted copy in the PDF viewer and search for the word you removed. It must find nothing.
- Select all the text on the page (click into the page, then Ctrl+A or Cmd+A) and look at the selection: nothing should highlight inside a box.
- Open the copy in Edit PDF and click on a box. There should be no paragraph under it to edit, and no picture to move.
- Open Edit PDF properties and clear the title, author, subject and keywords if they say anything the page no longer should; the pages are redacted, the properties are a separate list.
Scans, comments and the places people forget
- Scanned pages
- A scan is one picture, so a box removes that part of the picture, pixel by pixel. To find text on a scan by searching, run OCR PDF first; the words found under a box are left out of the text layer too.
- Comments and notes
- A sticky note, a highlight or a text comment that a box touches is removed whole, with its pop-up text, which would otherwise keep what the page no longer shows.
- Links
- A link under a box is removed, since its address can name what was there (a mailto: link carries the address itself).
- Form fields
- A filled form keeps its answers in the fields, not on the page. Run Flatten PDF first so the answers become page content, then redact.
- Earlier versions
- Some programs save a PDF by appending to it, so old versions stay inside. The redacted copy is written afresh and carries only what the pages show now.
- Bookmarks and the file name
- The outline (bookmarks) and the name of the file are not on the pages. Read the outline in the viewer, and rename the file on your device.
What it cannot do
- Decide what is sensitive. The patterns find addresses, phone and card numbers; names, amounts and places are yours to mark.
- Redact what it cannot see. A word split oddly by the program that made the PDF, or text drawn as a picture without OCR, is not found by search; draw the box by hand.
- Undo after saving. The removal is the point. Keep the original.