Redaction means permanently removing information from a document so that it cannot be recovered. A properly redacted PDF has the sensitive content destroyed — replaced with a black box — not just visually covered. The distinction matters enormously: many documents that appear redacted have the original text still accessible underneath, leading to significant information security failures.
False redaction: the most common mistake
The most common redaction failure is covering text with a black rectangle drawn on top of the page without removing the underlying text layer. The result looks correct when viewed normally, but the original text is still in the PDF file. Selecting all text in Adobe Acrobat, copying to a text editor, or using a simple PDF text extractor reveals the 'redacted' content in full.
This has caused real intelligence failures (the FBI's Robert Mueller memo, released in 2004, had redactions that could be bypassed by copy-paste), legal disasters (sealed court documents with recoverable content), and medical privacy breaches.
True redaction burns the text layer off the affected pages: the content is rasterized (converted to a flat image) and the text objects are removed. The resulting PDF contains no text data in the redacted regions — only pixels.
How Filum's Redact PDF tool works
You select text or regions on each page using the search interface or by drawing boxes. Filum replaces each selected region with a filled black rectangle at the PDF content level — the underlying text objects are removed, not just covered. The result is a PDF where the redacted content cannot be extracted by text selection, copy-paste, or PDF parsing tools.
The redaction runs entirely in your browser. Sensitive content is not uploaded — the document and the redacted areas are processed locally, then the PDF is written and downloaded to your device.
Verifying that redaction worked
After redacting, verify the result before sharing. Open the output PDF in any PDF viewer and attempt to select text in the redacted regions: place the cursor in a black box and try to drag a text selection. If no text can be selected — if the cursor passes over the black area without highlighting anything — the redaction is genuine.
For critical documents, use a PDF text extraction tool to confirm no text is present: run the output through a PDF-to-text converter and verify that the expected sensitive terms do not appear in the extracted text.
Never rely on visual inspection alone. A document that looks redacted but fails this test is not redacted.