Combinatorial Number Theory: Results of Hilbert, Schur, Folkman, and Hindman by Yudi Setyaan. A Thesis submitted in partial fulfillment of the requirements of the defense of Master of Science in the Department of Mathematics and Statistics. Simon Fraser University, July 1998I printed it out and still have it. It was not helpful for what I wanted, but it was interesting and I'm glad to have it.
Recently I wanted to email it to someone else so I searched for it again. Its gone! Now you have to pay for it at amazon. I also looked for the author on line to see if he might email me a copy (I doubt he gets any money from it and I suspect he would be delighted to find out the someone actually read it.) Couldn't find the authors email address, though I am hopeful that I will.
Moral of the story: If you find a document on line that you may want to keep, DOWNLOAD IT. Do NOT depend on it still being there later.