My supervisor gave me a pdf copy of an article from one of the Nature journals. This discusses in broadest terms the citation policy, but I am unclear how it relates to my case. Is it OK for me to post a link to the pdf, or can I only post the DOI, or is it irrelevant because anyone in a position to answer my question will have access to the databases and not need the pdf?
Disclaimer that this is not "official policy", just what I think is a common-sense solution.
As you are undoubtedly aware of, the sharing of scientific information is a ethical grey area and is still being debated hotly. (And hence you asked this question!)
If you want to be very safe, the best option is simply to quote any relevant portions of the article in your question, or a summary of it. If you provide a full citation to the original paper, most people who work in chemistry (and perhaps even some people who do not work in chemistry) will be able to find the paper in question, if any more details are required.