Thank you Mat, it is quite rewarding to get newer stuff working with old.
Perhaps noc Knezzen should share his project with you and see if there is something that can be done easily. The Bittorrent Client he did is working very well.
I have used MacTorrent myself, it is a very cool piece of software. I had to pinch myself when it actually worked!
I would be open to the idea, I wonder how far he got with it. Sadly I think serious work will be needed to get anywhere.
In terms of a true solution to PDF rendering on Mac OS 9, I am not sure if the best solution is trying to update GhostScript and MacGSView, or maybe trying to get poppler
https://poppler.freedesktop.org/ to build on Mac OS 9... hopefully its dependencies are portable. I recently came across the Retro68 toolchain, which I think will be easier than trying to use CodeWarrior with modern C++.
If poppler can be built, a drag and drop conversion utility (new PDF to old, or postscript) would be possible. I did something similar for the 'Drop ALAC' utility (although the code I based that on is a lot simpler than poppler). This seems like the best path to me, creating a new reader as good as Acrobat 5.0 would take far too much work.