Hi Martin,
we may be able to hide the download button at the bottom, but the other action bar comes from the installed tool which displays the content. Excel and probably Word documents are converted to pdf and your PDF reader renders it.
Take a look at my screenshot, I have a completely different tool bar.
If you really want to prevent the download/distribution you will need to check out Azure Information protection or similar solutions.
https://docs.microsoft.com/en-us/azure/information-protection/
With this you can even prevent making screenshots from the displayed files, also I don't have an idea how this works. Ok, you could still take a picture with your mobile, but that's beside the point:)
Of course this comes with the typical drawback, the higher the security the lower the user experience. You will lose the preview functionality and maybe you won't be able to edit these protected documents directly. I haven't tested it though.
Best regards,
Daniel