When you are dealing with other people's computers (especially desktop) I've run into Acrobat Reader v.5, 6 out in the wild, older versions of FoxIT Reader, Sumatra and many Linux Distro's PDF Render's. It isn't as bad as it used to be, I would be careful to assume your experience is true for everyone's machine.
Hey Kristal,
The must PDFs I've had open in Drawboard PDF is around 10, so I haven't experienced that, but I tend to close the PDFs once I reviewed, read, commented, plagiarized, etc. . It would surprise me though, depending on how the utilizing the Memory Controls with WinRT they can set their Cache to maximum size, and as a researcher I'll assume some of your PDF files are quite large. Typically mine are between 60K to 280MB.