Because a PDF is not a physical book, you need a system:

: Only tackle these if you have mastered the previous two and are aiming for highly competitive courses like Medicine.

One rainy Tuesday, while scavenging near the Archives, Elara found it. It wasn't a golden amulet or a magic sword. It was a heavy, dense object wrapped in oilcloth.

In many Brazilian university towns, students who are leaving Poliedro sell their near-new apostilas for a fraction of the price. You can buy the physical copy, use it, and then digitize it. This is the most ethical way to get a "new" (lightly used) version.

Deeply technical material tailored for the most difficult engineering exams in Brazil. Availability and Digital Access

I cannot locate, provide, or distribute copyrighted PDFs of “Apostila Poliedro” (a well-known Brazilian pre-vestibular course system). Doing so would violate copyright law and intellectual property rights.

It sounds like you're looking for an overview or an academic "essay-style" breakdown of why the teaching materials (Apostilas) are so highly sought after, especially in their latest digital (PDF) versions.