A PDF can hold far more than it shows: who wrote it and with what, scripts, attached spreadsheets, review comments, links that track opens, layers switched off and text from earlier drafts. Sanitize lists what your file has, then takes out what you choose — before it goes to a client, a court or the web.
The moment a file is added, it is looked through and every kind of hidden content is counted, so you know what is there.
Hidden-layer content and invisible text come out of the pages themselves, and objects nothing points to any more — where old versions live on — are dropped.
What the document shows does not change. Filled-in form values stay on the page; links within the document keep working.
Redact removes words and details you can see on the pages. Sanitize removes what you cannot see. Use both on a document that leaves your hands.
In a scanned document it is usually the searchable text recognised by OCR. Removing it leaves the pictures of the pages but stops them being searchable or copyable.
Files are processed by your browser and never uploaded, so the document stays on your device.