Sharing zipped files of PDFs amongst people is quite possibly a copyright violation. Do you have distribution licenses for all the material you ar referencing?
I am curious as to how you share PDF files with others ensuring that you have distribution licenses for all of the files. Not even RightsLink from the Copyright Clearance Center automatically purchases those distribution licenses. What I do know is that other ways of sharing files do not have the same privileges of communication applied to e-mail in the law, so that is the reason why this is still the most commonly used strategy.
