PDF security guide
How to Permanently Redact a PDF
A black rectangle is not enough if the original text remains underneath. Learn how flattened redaction works and how to test the output before sharing it.
Why ordinary black boxes can be unsafe
A PDF can contain separate layers of text, images, annotations, and drawing commands. Placing a black shape over a name or account number may hide it on screen while leaving the original text object intact. Someone may still select the text, copy it, remove the shape, or recover it with an extraction tool.
PDF Plain avoids that pattern. When a page has redactions, the browser renders the page to an image, draws the black areas into that image, and puts the flattened image into the output PDF. The original text layer from that page is not copied. Pages without redactions remain normal PDF pages so their text stays sharp and searchable.
Step-by-step
1. Work from a private original
Keep the source file in a controlled location and make a copy for redaction. Unlock a password-protected PDF with the known password before editing it.
2. Cover every sensitive item
Add a redaction rectangle, move it over the content, and resize it past the full edges of the text or image. Check headers, footers, barcodes, signatures, and repeated identifiers on other pages.
3. Choose output quality
Standard 150 DPI is usually enough for screen viewing and produces a smaller file. High 300 DPI keeps scans and small text sharper but can make affected pages much larger.
4. Download the flattened copy
The browser processes affected pages one at a time and builds a new PDF. The original file remains unchanged on your device.
5. Verify before sharing
Open the result separately. Search for the hidden terms, try to select text on redacted pages, and visually inspect every box at high zoom.
A practical redaction verification checklist
- Search: use Ctrl+F or Command+F for each hidden name, number, or phrase. It should not be found on a flattened page.
- Select: drag across the redacted area. Text on that page should not be selectable because the page is now an image.
- Copy: try copying nearby content into a plain-text editor and make sure the secret value is absent.
- Inspect: zoom in to confirm no letters, digits, or image edges remain visible around a box.
- Review properties: page redaction does not guarantee that filenames, titles, authors, comments, attachments, or other metadata are cleared.
- Keep the original private: only the verified redacted copy should be sent to the recipient.
For high-stakes legal, medical, government, or regulated disclosure, follow your organization's approved redaction process and have another person review the output.
What flattening changes
Flattened pages lose selectable text, vector sharpness, links, form fields, and accessibility structure because each affected page becomes an image. Existing digital signatures can become invalid when page content changes. File size may also increase, especially at 300 DPI or when many pages are redacted.
These are deliberate tradeoffs for removing visible page content safely. If only one page contains a secret, the untouched pages can retain their original text and vector content. Before redacting an entire long document, consider whether you can extract and process only the pages that need to be shared.
Common questions
Does flattening remove the hidden text?
On redacted pages, the output contains a rendered image with the black boxes baked in rather than the original page text objects.
Does this remove metadata too?
No blanket claim is made. Review document properties, filenames, comments, and attachments separately. Use Edit PDF Metadata when appropriate.
Can I redact scanned PDFs?
Yes. Scans are already image-based; the redaction is baked into the newly rendered page image.
Why is the output larger?
Images often require more storage than vector text. Choose 150 DPI unless the recipient needs print-quality detail.