What makes a document "confidential" from a tooling perspective
NDAs, unsigned contracts, patient records, and financial statements share a common property: showing them to an unauthorized third party can itself be the harm, regardless of what that third party does with the file afterward. An upload-based converter is, technically, an unauthorized third party the moment the file lands on its servers — even before any human looks at it.
Converting locally in the browser removes that step entirely: there's no server in the pipeline to be a third party, so the confidentiality question doesn't arise in the first place.