I just tested 8 single pages pdf's. It consumes in total 30.1MB VRAM here. I have a small monitor atm, so I run only 1024x768 24 bit now. If I had a bigger screensize I'd probably have used more than that.
Luckily I have MOS2.5 and 64MB VRAM, so I did open 9 more pdfs and was then at 47.1 MB.