Sketchup 6 Pro Trial cannot be installed side by side with the free version. When you run the pro installer it finds the free version, even if its installed in a different directory, and performs an ‘upgrade’. I assume it does this by querying the registry, looking for an existing install. I tried a couple of other attacks like running the ‘repair’ option from the free version after the pro install, no dice.
So if your a hobbyist and want to continue to be able to export PDF and other files by using the Sketchup Pro trial you have to be a bit more creative. You could get a friend in college to register a copy for you, the educational license is only $50 per year. You could run Sketchup Pro in a virtual machine to keep it from detecting the free version, VMWare is free.
I wish there was a pricing option for hobbyists and others interested in producing non commercial and Creative Commons works. A price similar to the academic license would at least attract people that don’t do CAD for a living. $50 annually for anyone and just stipulate that it cannot be used for commercial purposes. Sketchup is a program I would gladly pay money for. The Sketchup team makes a great product and they deserve support. But I’m not going to give up almost $500 (50% of my annual hobby budget) just so I can export my drawings to PDF.