I think its fair to say that a lot of CAD users including your own office will have at some point accessed such files, protected or not, for possibly genuine reasons. AC's excellent PDF conversion isn't really the issue as you can still trace the original from vector or raster PDFs or even TIFFs and what you are asking is probably impossible to implement as you are effectively saying the drawings which are a matter of public record should be unreadable unless you have the permission of the author.
Perhaps the ideal solution is to try and give your client best value so they don't go elsewhere for the next stage!
Apple iMac Intel i9 / macOS Sonoma / AC27UKI (most recent builds.. if they work)