I came up with a work-around that may help others who find themselves in a similar situation in the future:
I changed the folder where I store PDF files to be directly under the root folder. Then I was able to restrict the access of the Procurement group to just that folder. To be accurate, I did the same with STP, DXF and STL files, too, and restricted Procurement to see only those 4 folders.
It works, but it does seem like a work-around, so I would still be interested to know how I could achieve my original aim.