[jocampo]

Really difficult to tell, almost impossible.

You can put watermarks as a comment on the JPG image or even on the CSS classes, or as part of the HTML tags, inside the name of the directories and files, direct alteration of the zip file, etc.

I would not remove the DRM, is illegal in USA and several other countries, but if you do, I would not expose those books at all, not even dropping or putting that on Dropbox folders.

The most secure way to protect an electronic document or device from hackers is just unplug the device from the network: not sharing at all. Even in your intranet, if someone breaks your firewall or get into your house's network, whatever is stolen and later reproduced or posted, the initial owner is 100% responsible for the copyright violation (assuming there was a watermark, of course)

By the way, the Dropbox transmission is encrypted, the communication itself, but not the folder or data. If the server is compromised, everything will be compromised too.