Query about CPU and GPU in Toshiba X875-Q7190 laptop

I have a Toshiba X875-Q7190 laptop. It has a GPU built into the CPU and a discreet GPU. With some fies, particularly PDFs, it takes a tremendous amount of time to render successive pages. My thought is that it’s utilizing the CPU-based GPU. How can I get the laptop to always use the discreet video card?
