Not a solution per-se, but perhaps a work around…
Before the documents are converted to PDF, include a large-bold diagonal watermark that says Proprietary and at the bottom of each page include another watermark with the copyright restrictions/caveat, etc.
Of course, the documents could always be hand-copied, but then the miscreant would indeed be at some legal peril.
Harv 27" i7 iMac (10.13.6), iPhone Xs Max (12.1)
Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
