Which information? The server is then not reproducing/transmitting/redistributing the content, only indices into the content. I don't see why this would be copyright infringement.
Sure, it's not, so do not take it as advice, especially not ultimate advice. However, I find the inevitable intellectual shutdown when discussing matters like this even more repugnant and unwarranted.
Edit: How about the backend just returns pointers to the text (word #x till word #y) and the js just (re)assembles it?