Here's my guess, assuming your Mac has an integrated GPU (eg Intel Iris Graphics).
When you have your thesis open in Preview, graphics card memory is used to hold the image ("texture") of the Preview window, and perhaps also some off-screen-but-decoded pages from the thesis.
With an integrated graphics card, the video memory is actually (partially?) located in system RAM, which is shared between the CPU and GPU. On some integrated graphics cards, the amount of system RAM used is dynamically allocated (see Apple HT204349).
I'd guess that you are intermittently seeing a bug in the graphics card driver, which isn't releasing system memory correctly when Preview reloads your thesis PDF. (However this bug is mitigated by OS X / the driver correctly releasing the memory when Preview quits.)
You could try looking at the output of kextstat and see if the numbers in the Size column increase when you experience the issue. My theory is that the 8GB increase you mention will be due to the graphics card driver.
The following command (from a comment on this related and interesting answer) sorts the output of kextstat to make it easier to see which kext is using the most memory (although note this sorts by the Wired column... there's a similar, simpler incantation in this answer with an explanation if you'd like to tweak this).
kextstat | awk 'NR==1{ printf "%10s %s\n", $5, $6; } NR!=1{ printf "%10d %s\n", $5, $6; }' | sort -n