I was using ProcessMonitor to diagnose a weird issue with FARR. A pdf I received from a vendor had a space as the last part of the file name before the ".pdf"
FARR would display the name but without a space (which I didn't notice - hence ProcessMonitor) and try to launch it the same way via quickdoclauncher. So I think it's an issue within FARR rather than doc launcher.
However, there was no popup or alert saying that the file was unable to be located... which is odd because with no index, the file should always be there.