Here are the legitimate avenues to find the PDF:
Ejerció la cátedra en instituciones de altísimo nivel como la Universidad Nacional Autónoma de México (UNAM), el Instituto Politécnico Nacional (IPN) y el Centro de Investigación y de Estudios Avanzados (CINVESTAV).
While less likely for a technical treatise, Project Gutenberg’s Spanish volunteers occasionally upload classic scientific texts. It is worth a periodic check.
: The "treatise" style can be intimidating for beginners or those looking for a quick, casual review. It requires a significant time investment to work through.