Why black boxes don't redact a PDF
A black box hides text from your eyes. It does not take the text out of the file.
A PDF page is built in layers. The words are stored as text, and anything you draw afterwards, such as a rectangle, a highlight or a picture, is stored as a separate object placed on top. When you cover a name with a black box, the file now contains the name and a box. Anyone can select the area, copy it and paste the name somewhere else. Search finds it. So does an AI tool that reads the file.
This is how names and figures leak from court filings, released reports and HR letters: the document looked redacted on screen, and nobody checked what was still inside.
Check a PDF or image with the PDF Redaction Checker. It runs in your browser and the file is not uploaded.
Common mistakes
- A black rectangle drawn over the text. The most common one. The shape is on top; the text is underneath.
- Black highlighter. A highlight is an annotation. Remove the annotation and the text is back.
- White-out, or a white box. The same problem in a different colour.
- Changing the text colour to white or black-on-black in a word processor before exporting. The words are still text.
- Redaction marks that were never applied. Some editors let you mark an area for redaction and apply it as a second step. Marked is not redacted.
- A blacked-out scan with a hidden text layer. Scanners and OCR add invisible text behind the picture so it can be searched. Black out the picture and the hidden text still has the words.
- A see-through box. A dark, semi-transparent cover dims the text but leaves it readable when the contrast is raised.
- Cropping the page. Cropping changes what is shown. The content outside the crop is usually still in the file.
- Forgetting everything that is not on the page: the author name and title in the file's properties, comments, tracked changes and attached files.
How to check a file yourself
- Open the PDF and press Ctrl + A (Cmd + A on a Mac) to select everything, then copy.
- Paste into a plain text editor.
- Search the pasted text for the names and numbers you meant to remove.
- Open File, Properties and read the title, author and subject.
- Look for comments and attachments.
The PDF Redaction Checker does the first three steps for you and shows where on the page each covered piece of text sits, including covers that are not black.
How to redact properly
- Use a real redaction tool and apply it. A true redaction deletes the text and replaces the area, so there is nothing underneath.
- Or turn the page into a picture. Black out the area, then save each page as an image, so no text layer remains. The text can no longer be selected or searched, which is the point.
- Remove the extras. Clear the file's properties, comments and attachments.
- Check the result before you send it, with the steps above.
The same applies to screenshots and photos of documents: a blur or a thin scribble can sometimes be read through, so cover the area with a solid block and check the saved image, not the one in your editor.
Related tools
PDF Redaction Checker · Sensitive Info Finder for PDFs · Image PII Redactor · Mask Aadhaar and Indian ID numbers