I still need to go through and catalogue my PDF collection. I'm thinking that will dwarf my hard copy collection. Not quite as large as yours though. Looking at just your 5E list, I'm amazed you have so many PDFs for that alone!
PDF bundles suck me in too. I own several Humble Bundle and Bundle of Holding RPG bundles that I've barely even had a chance to flick through, let alone read.
Legendary Games has their 20-for-$20 bundles (I'm using The Murmuring Fountain and some of their Gothic Tomes from these bundles in my 5e Carrion Crown game I'm running right now), Cubicle 7 and Ponyfinder were from Bundles of Holding, and Humble Bundles provided Frog God, Kobold Press, Total Party Kill, and Troll Lord, while Fat Goblin puts out their own fantastic bundles as did a bunch of DM's Guild specials. I keep my eye out regularly for ones that interest me and pounce.
There are also a lot of free and PWYW titles.