Redact a PDF: black out text for good

What really happens to the page

A PDF page is a set of drawing instructions, with its text as text. A black rectangle drawn on top hides a word from view but leaves it in the file, where search, copy, screen readers and anyone who deletes the rectangle can still find it. That is how redaction goes wrong in the news.

Octoolo takes a different route. When a page has a Black out mark, the whole page is drawn as a picture, at 2.5 pixels per point (180 dots per inch), with the black boxes painted into it, and saved as a JPEG at 90% quality. The page's original instructions, its text, its pictures and any form fields that sat on it are removed from the file, and the page object itself is replaced, so a reader of the saved file finds only the picture. Pages without a Black out mark are kept exactly as they were, text and all.

What changes on a blacked-out page

• Its text can no longer be selected, searched or read aloud, anywhere on that page, not only under the black. A reader who needs the rest of the page as text can use image to text on it.
• Form fields on that page disappear, along with anything typed into them. Fields on other pages stay.
• A turned page is saved upright, the way it looked on screen.
• The page looks like a scan: sharp at ordinary sizes, pixels when zoomed far in. The file usually grows a little.
• Links, bookmarks and the document's title and author are left alone. If the author's name or the file's title gives something away, change it elsewhere before sending.

White-out is not redaction

The PDF Editor also has White-out, a white box for tidying a page: hiding a stray mark, a stamp or an old date before you type a new one with Text. It is drawn on top, so the words under it stay in the file. Use White-out for appearances and Black out for anything nobody must find.

The same goes for other programs' black rectangles, markers on a scan and cropping a screenshot of the page: unless the tool says the text is removed, assume it is still there. The safest check is to open the saved PDF and try to select or search the blacked-out words.

Before you send it

Save the redacted copy under a new name and keep the original where only you can reach it. Then open the copy, search for the word you removed, and try selecting across the black box. Look at every page where the secret could repeat: headers, footers, a reference number, a signature block, the metadata of a scan. If the whole document must be unreadable as text, black out something on every page, or turn the pages into pictures with PDF to JPG and make a new PDF from them with JPG to PDF.

For documents you redact often, the PDF Editor also deletes pages, extracts the ones you can share and adds a watermark such as REDACTED COPY.