I was actually searching for that PDF myself a while back. The title confused me at first because 'El Diablo Ilustrado' isn't one single novel you can just download. It's a collection of short stories by the Chilean writer José Donoso, published in the 70s. The 'Ilustrado' part refers to the illustrations that were in the original edition, which you might not even get in a scanned PDF floating around online.
Trying to pin down a single plot is tricky. It's not a continuous narrative. It's more like a dark, surreal tour through various twisted scenarios. The common thread is this exploration of duality, of masks people wear, and the grotesque hiding beneath polite society. Think of it as a series of unsettling character studies, often with a satirical or fantastical edge, where the 'devil' is more a metaphor for hidden desires and societal rot than a literal horned figure.
If you find a PDF, expect stories about aristocrats whose lives are falling apart in bizarre ways, or mundane situations that slowly curdle into something strange. The tone reminds me a bit of other Latin American greats from that boom period, but with Donoso's own particular chill. It's not a breezy read, but the imagery sticks with you. I ended up buying a used paperback copy because the PDF I found was a mess of poor scans and missing pages.