"Un"rendering of PDF pages

I have an 64GB iPad 4 on iOS10 and the latest version of Scrivener. My project isn’t particularly big (4.4MB in total) with 33 small RTFs (average 6KB each) and 4 PDFs, in particular one PDF has problems when showing images:

When I use the quick reference on a PDF while editing (see above), after a minute or so the figures in the PDF go black. I can still scroll the PDF and the pages with only text show, but the images do not render. If I close the PDF then open it again the images are shown but a little while later they can then go black again. Swipe closing all other running apps doesn’t fix this. I’m not sure if this is just a limitation of the memory architecture of iOS but that PDF itself displays just fine using PDF Expert with Scrivener also running without issue.