There are many PDFs (Portable Document Formats) of many math or physics books available in online. But I am not sure whether those are legal/authorized or not. If they aren't, then the readers of these PDFs (Books) are just reading these without paying for the authors' hard work. I would feel really bad to do that. So, my questions to you guys are: \(1.\) Are these PDFs legal/authorized? \(2.\) If they aren't, where can I learn math and physics best? Pleeeze give me as much 'best weblinks for learning math and physics' as you can. \(3.\) If only some of them are legal, is there any way to find which of them are legal? If you guys know about such PDFs, then pleeeze give me the links. Thanks in advance.