Universities like MIT, OpenStax (Rice University), and the University of Georgia release free course materials. Search for:
If you are looking for a complete textbook or detailed lecture notes, these sources are widely used in engineering and mathematics: Kern and Bland's " Solid Mensuration plane and solid mensuration pdf
In an age of cloud computing and interactive apps, you might ask: Why specifically a PDF? Universities like MIT, OpenStax (Rice University), and the