It's usually not that the authors don't want to provide PDFs but with scientific journals you're often asked to sign a form that transfers the copyrights to the journal. In other words, you're not allowed to make the paper available even though you're the author.

In my experience, finding out when a paper was published is often not too difficult. In most cases, Google makes it easy to find the Journal issue and/or the conference proceedings of the paper. Or you find some third paper that contains a reference with date information that you can then use to double check.