Why black boxes in PDFs fail
A PDF stores its text and its drawings separately. A black rectangle added with a comment or drawing tool is one more object on the page, painted after the text. Readers see a bar. Anything that reads the text layer (copy and paste, search, screen readers, text extraction tools) still gets the original words.
Where hidden text survives
- Under annotations. Rectangles, highlights and stamps can be moved or deleted by anyone who opens the file.
- In the OCR layer of a scan. A scanned page often has invisible text behind the image so it can be searched. Covering the picture leaves that text alone.
- In form fields. A filled field keeps its value even when a box is drawn over it.
- In earlier versions. PDFs can be saved incrementally, appending changes to the end of the file, so an older copy of the page may still be inside.
- In bookmarks, link targets, attachments and properties. None of these appear on the page, and all of them can hold names or addresses.
Test a redacted PDF in two minutes
- Open it in a browser or PDF reader, press Ctrl+A (Cmd+A on a Mac) and paste into a plain text editor. Anything readable there is still in the file.
- Search the PDF for a word you removed, such as a surname.
- Open the document properties and look at the author, title and keywords.
- Check the attachments or bookmarks panel if your reader has one.
Redacting so it holds
A dedicated redaction tool removes the content under the mark instead of adding a layer. AnyRedact goes one step further and rebuilds each page as an image on download, which drops the text layer, form fields, links, attachments and metadata in one go. How to redact a PDF walks through it.