Your question got me curious to see what's going on, as I have a lot of old Windows PMK's laying around from years past as well.
First, a PMK cannot be opened as an editable file (to my knowledge) - it can only be dragged onto a layout (or inserted there). The layout onto which it is placed can then be saved as a DWG.
Dragging any of my old circa 2001 / 2002 PMK files into any of AC 10 through AC 20 pops up a message saying that only PMK files created with PlotMaker 3.0 or newer can be placed on a layout.
Digging into my old Snow Leopard boot clone, I see that AC 7 used PlotMaker version 2.3; AC 8.1 used version 3.1 and AC 9 (final PlotMaker) used version 9.0.
So... if Mac is your only option, it would appear you would need the capability to reboot into the old Snow Leopard OS in order to run AC 8.1 or AC 9 to convert the PMK files using the version of PlotMaker included in those versions. Those versions of AC were written for the old PowerPC Macs... and Snow Leopard allowed Intel Macs to emulate PPC code using what was called "Rosetta".
If you don't have the ability to run Snow Leopard - you might have to find an ancient Mac or an old Windows PC (or virtual machine) in order to run the Windows version of PlotMaker included in 8.1 or 9 to do the conversion ... or re-open the original PLN in those versions of AC and republish the PMKs.
For all of the work... if they just need to-scale line work and you have old PDF files that were created with vector-based line work (vs scanned images)... you or they can use a variety of tools to convert the PDF to snap-able/trace-able, and (slightly) editable line work.
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AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB