The mistake that keeps happening
There is a type of blunder that reaches the news every few months: an authority or a company publishes a document with names and addresses covered by black bars. Hours later the comments quote exactly what was underneath. Nobody cracked the file. The text was never gone.
The reason is simple. A PDF is built in layers. Draw a black rectangle over something and you have added a layer. The text underneath stays fully intact. You can select it, copy it and paste it, and in many programs it is enough to drag the rectangle aside.
Covering versus removing
Covering means something lies on top. Removing means the characters are deleted from the file. Only the second survives scrutiny.
The test takes ten seconds: open the redacted file, drag your mouse across the area above the black bar and paste it into any text field. If the text you wanted hidden shows up there, it was only a bar.
What else is inside the document
Even with the visible text properly removed, a second trail often remains: the metadata. Depending on the program it holds the author's name, the computer name, the creation date and sometimes the full file path, which can give away project or client names.
Passing a document on anonymously means dealing with both. Our redaction tool deletes the marked passages from the file and clears the metadata along with them. To inspect metadata on its own, use Remove PDF metadata.
When redaction is actually needed
The everyday cases:
Bank statement as proof of income. The landlord needs the salary line, not every single debit.
An invoice as a receipt. The amount and the date matter, the full customer number does not.
Certificates and job applications. Date of birth and home address do not belong in every application.
Passing files to third parties. The moment a document goes to someone who does not need to know everything, the question answers itself.
A word on completeness
A detail rarely appears in one place only. A customer number shows up in the letterhead, in the footer and in the subject line. Redacting the most obvious spot achieves nothing. Before sending, read through the finished document once, ideally using the search function for the number or name that was meant to disappear.
In short
A black rectangle is decoration, not protection. The detail is gone only once the characters are deleted from the file. Metadata counts too, and the final check is select and paste. Processing happens in the browser, so the document never leaves your device.