Forgotten Books republishes thousands of classic works that are in the public domain. Forgotten Books offers all of their titles as free PDF downloads and provides links to ePub versions of the titles. If you desire a higher quality resolution for your PDFs, Forgotten Books offers those to their paying members.
Applications for Education
When my students and I read Red Badge of Courage together I was one copy short of what I needed so I jumped on Google Books and found a copy that I could print chapter-by-chapter to use in my classroom. I used Google Books to find a copy of that public domain work, but I could have just as easily used Forgotten Books to get an ecopy of the Red Badge of Courage.