Use tools that process the file in your browser rather than on a server, strip metadata before sharing, and be aware that the obligation is usually contractual as well as personal — a client document may not be yours to upload at all.
Most advice about confidential documents stops at "be careful". That is not a procedure. What follows is one — the checks worth making before you open a sensitive file in any online tool, and the two places confidentiality usually leaks that have nothing to do with converters.
First: is it yours to upload?
This question comes before any technical consideration, and it is the one most often skipped. A document belonging to your employer or a client may be covered by a confidentiality clause, a data processing agreement, or a policy that names which services are approved.
If so, uploading it can be a breach regardless of how reputable the service is. The service being secure is not the same as you being permitted to send it there.
Second: does the tool need the file at all?
For most document work — converting, merging, splitting, extracting text, changing an image format — a modern browser can do the job without a server. When that is true, the confidentiality question dissolves, because no transfer occurs.
Verify rather than assume. Load the page, disconnect from the network, then run the operation. If it completes, the file never left your device. There is a fuller explanation in what actually happens when you upload a file.
Documents carry more than their visible content, and this is where confidentiality most often leaks — not through a converter, but through the file you deliberately sent.
Metadata
A Word document records its author, who last modified it, the company name, the revision number and how long it was edited for. A proposal sent to a client can quietly reveal that it was drafted from another client’s template, and by whom.
Comments and accepted changes remain in the file even when the view is set to show the final version. A recipient can switch that view. Anything said in a margin comment should be assumed readable.
Content that is hidden rather than removed
Text covered by a black rectangle is still text. Drawing a box over a paragraph in a PDF hides it visually and leaves it fully selectable underneath — a mistake that has embarrassed governments and law firms repeatedly. Deleting the content is the only reliable redaction.
Cropped images
Cropping in Word or PowerPoint usually hides part of a picture rather than discarding it. The original can often be recovered — which is worth knowing if you cropped something out for a reason. Extracting the images shows what is really stored.
Fourth: control the copy you send
Once a document leaves your hands you have no further say, so decide deliberately what form it leaves in.
Send PDF rather than the editable original when the recipient only needs to read it. It carries less recoverable history.
Send only the pages that are relevant — extracting pages is quicker than explaining later why the rest was included.
Password-protect it when the channel is not under your control, and send the password separately.