Hmm, you’re right - @brad’s licensing model allows you to install C3 on multiple machines, but only run it on one at a time. Since you DO need Performer for my proposed solution (you need song states, which are only available in Performer), this would be a pretty expensive solution, just to pop up PDFs
You could take a look at bome’s MIDI Translator Pro - looks like it can also execute programs based on MIDI triggers. But it’s not free - it costs 59 EUR.
Depending on the content of your PDF documents, there might be an alternative solution to displaying PDF documents: not sure if you’ve looked at LivePrompter yet? It’s a (free) teleprompter specialized for live musicians, but it doesn’t display sheet music, only lyrics and chords. But it works really well with Cantabile and allows exactly your scenario to work nicely. So if it’s mainly lyrics and chords you need displayed, give it a try!
Cheers,
Torsten