So do you want .PDF output or .PLT output? In order to output to a PDF on a paper size which you don't actually have a device for you will need to install a print driver for a device that has your required paper size or install a PDF print driver. There are several people here on the forum who use different PDf print drivers (you can search the forum for them), I personally use PDF XChange'd PDF Printer for my PDF output as it gives you tons of options especially when batch plotting from Datacad. the quick layout must be set to locate the paper over your drawing area so it knows what to print. Sounds like you might want to brush up on printer in the Datacad manual as well which you can get from the main
http://www.datacad.com website if you need to download a copy.