Best PDF security option

The only reliable way to do it is with DRM technology, but that will cost you a pretty penny.

Alternatively, you can indeed add a watermark and then digitally sign the file to prevent it from being edited (at least, you will be able to detect such editing, as it will invalidate the signature). You can also apply a security policy, especially the one that requires a password to open the file, as the encrypts the file's entire contents, making it more difficult (although not impossible) to get around it. All the latter can be done using Acrobat. The former requires specialized tools.

There is no way you could stop this. There is security you can set, but a lot of apps ignore it. Also, if it's worthwhile, anyone could photograph the pages and republish it in a few minutes.