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