The Zombie Hack is another adaptation of The Black Hack by Bloat Games.
The pdf is 38 pages counting covers, legal text, dedications/acknowledgements, and such.
I discussed The Black Hack rules in my Vigilante Hack review, but here's another summary.
The rules are a stripped version of OD&D. There are a few new rules (such as Usage Die) and some familiar thing from modern games (Advantage/Disadvantage). Players do all of the rolling. Armor is damage reduction. This also has the Bloat Games Hack variants, such as multiple advantage and disadvantage. These are the classes: Fixer, Leader, Scavenger, Slayer, Survivalist, and Tracker.
The enemies section of the book is full of the obvious... zombies!!! We're presented with rules for infection and 7 different types of zombies which do a great job emulating zombies from a variety of movies (such as Night of the Living Dead, 28 Days Later, and Return of the Living Dead). There's also a bonus pwyw zombie that you can grab here.
The book ends with a short piece of in-universe fiction.
You can buy The Zombie Hack here. There's an alternate cover version too.