I have a customer that asked me if is possible to limit authorization to a single library for some users. He wants them to just read the files in the library but to forbid downloading them. I couldn't find any option for that.
Out of the box, this isn’t possible. You can, however, restrict both download and printing with Information Rights Management for all users, but not for a group of users. And since there is a snipping tool and a print screen function on all operating systems, users can with a little effort still get the content of document and save it somewhere else. So this doesn’t make your document content protected, just less vulnerable.
You won't be able to do that even if you disable the "Download a copy" button. If a customer can open files in a client application like Word or any kind of PDF viewer, that user will be able to save the files from there.