You are correct that your printer is not drawing (much) power from the USB connector because it has its own power supply. However, sending print is an intense task.
The camera on the other hand, does draw its power to transfer pictures etc. from the USB port.
Now when you are talking about the printer handling .html and .doc but not PDF, you are probably dealing with a Driver issue, rather than a power from the USB port issue.
What PDF reader do you have installed? Adobe?
I almost always suggest that folks with low-resource machines uninstall Adobe (unless they have the paid version and use all the bells and whistles)... and substitute the excellent Foxit
pdf reader that does everything and more than the free version of Adobe........ and has a much smaller footprint (load on the machine).