This I am sure most people already know about. Personally I love to learn and the majority of my searches have reflected this. The majority of the time I use Google Scholar, Google Books, and all the free online libraries out. I also have subscribed to many academic you tube channels mainly colleges. Amazon is another great place to find free online learning materials with their large amount of free literature.
With all this I believed I had at the very least a grasp on where to find the knowledge available on the web. This was of course until I searched a topic while applying (PDF) in the search term. After doing this I was blown away with the massive amount of books written by well renowned people in all the various academic institutions. All of these are free to read, but remember simply because they are free and you are able to download them easily they are under copyright.
Make sure if your interested in a subject try placing pdf in the search term.