- Why a black shape is not the same as redaction
- Why marking, applying and saving are separate steps
- What to check after reopening the file
The short answer
Before sharing a PDF outside your organisation, you may black out names or numbers. But placing a black box on top does not always remove what is underneath; the text or image can still be in the file. When something must not be shared, use a redaction feature that deletes the text or image itself, not one that only hides it, and reopen the saved file to check.
Hiding and removing are different operations
A PDF file is built from layers of text, images, shapes and comments. Drawing a black rectangle with a shape or annotation tool is like laying a sheet of black paper over a printed page: the words underneath are still there. They may be invisible on screen yet still selectable, copyable or findable by search.
Covering part of a screen or document is often called masking, and visual masking alone does not remove the content. Redaction is a feature that removes the selected text or images from the file. Names and steps vary between PDF apps, and some apps have no redaction feature at all.
Treat marking, applying and saving as separate steps
In Adobe Acrobat Pro, redaction happens in roughly three stages.
- Select the text or images to remove and mark them (at this point they are only marked)
- Choose Apply and confirm the prompt to carry out the removal
- Save the redacted file
Adobe's tutorial describes the original file being left unchanged while a new PDF with "_Redacted" added to its name is created. Watch out for half-finished states: sending a file that was only marked, or applying redactions but never saving. In other apps, find out in advance which step actually commits the change.
It may still be selectable, copyable or findable by search.
Marking alone only flags the area. Finish with apply and save.
Information outside the visible text
Even after the visible text is removed, a file may hold other information.
- Searchable text (the text layer placed behind a scanned image)
- Comments and annotations
- Attached files
- Metadata (information about the file, such as author and title, shown in the document properties)
- Hidden layers
Adobe Acrobat Pro has a "Remove hidden information" feature that deletes metadata, comments, hidden layers and similar items, either all at once or item by item.
For example, if you redact the name column of an attendee list but a working comment saying "A is absent" or the original spreadsheet is still attached, the recipient can still learn the names.
How to check before you send
Save the edited file under a different name from the original. If you overwrite the original, you can neither start again nor check what was removed.
After saving, close the file and open it again. Then use a few checks as supporting evidence: search for a word you removed, try selecting and copying text around the redacted area, and open the lists of comments and attachments. A search that finds nothing does not prove the content is completely gone, though. First confirm that you finished every step your software requires to commit the redaction.
Do not upload documents containing confidential business information to online conversion or editing services without your organisation's permission. Sending a file out in order to redact it can itself mean sharing the information.
Summary
Looking black and being removed are two different things. Commit the redaction with a proper redaction feature, and check the information outside the visible text before you send.
Checklist
- Used a redaction feature that deletes content, not a black shape
- Applied the redactions, not just marked them, and saved
- Checked comments, attachments, metadata and hidden information
- Saved the edited file under a different name from the original
- Reopened the saved file and used search and copy as supporting checks
One thing to check today
Check whether the PDF app you use every day has a redaction feature that deletes information rather than only hiding it.