No files, no ownership, all hail our cloud overlords
You can't control what the person does with it or who they share it with. For all I know they are just uploading it straight to ChatGPT. Therefore sharing files is loosing ownership.
On the other hand just because something is in a cloud doesn't mean it isn't yours. It can also be in your own self-hosted cloud.
See also: analog hole
Similarly, these kinds of measures ("it'll make it harder which is better than nothing") usually just ends up annoying legitimate users while not providing any actual security.
Example: screenshot. Now toss it in ChatGPT or tool of your choice and ask it to extract the content.
Maybe they are feeding it into a screenreader or a local LLM because that is what works for them, or because they suffer from challenges you aren't aware of and don't need to be. You still have the same "ownership" of any files on your computer and data within, from a legal perspective. What you are talking about isn't ownership, it's control. Control to hamper others' usage of data which they're already allowed to access.
That desire for technical control is an exercise in futility: If I wanted to share a piece of sensitive info, aside from copying it via a number of technical means, I can just talk to someone in person and say "I saw...", or perhaps a malicious actor will hack my computer and gain access to it via screenshots, packet captures, arbitrary logging, etc.
Thus, if you don't trust me to secure the contents of a file (whether due to incompetence or malice), and aren't willing to risk it, don't send it to me. Most people in the world don't send me their files, and I'm ok with that. Alternatively, and also common, is to send the file to people who are contractually bound to use it in accordance with a set of terms. For example: an employment contract; or a partnership agreement.