Presumably because someone with the rights to distribute it, either you or your school, uploaded it to them. A more interesting question is, if these companies

Presumably because someone with the rights to distribute it, either you or your school, uploaded it to them. A more interesting question is, if these companies do well and stay in business a long time, won't it become increasingly difficult to write an original paper that isn't flagged for plagiarism? There's only so many ways to describe the effects of the Lend-Lease Act on postwar Europe.