if the scan itself is from 1665, then they don't have a leg to stand on.
That would be remarkable ;-)
The question is not whether the RS has copyright in the 17th-c journals. Nor is it whether they have copyright in reproductions of those journals in general. They couldn't argue for either of those!
However, they might be able to claim that the work involved in making a particular set of reproductions (e.g. those PDFs) is 'original' and therefore creates copyright.
I suspect they could but I'm not a lawyer either.
Chris