Well pdf search online can be tricky. There are many pdf files out there that are fake and may even contain virus. So you need to be very careful when searching for pdf files online. But why search for pdfs in the first place?
PDF files are document files which commonly contain research data and other information which are compiled online. These pdf files have been made because the author dies not wish for the usual internet surfer to copy paste his work or even change some contents to it.
PDF documents can be used for school research and the good thing about them is they can be accessed not only online but also on your mobile devices, as well as on standard video cameras and even still cameras. In general PDF documents are now popularly known as E-Books.
Most books including the famous Twilight Saga can be downloaded for free in pdf format. You have the choice to print it or read it in your computer. Through this you can save hundreds of dollars in actual purchase of the original book.
However not all E-Books or PDF files are free. There are those which charge modest fees and some even try to charge higher fees than the printer version. My recommendation is to really try to find what you are searching for before actually paying for a paid pdf file.
Hope this helps.