This is sample2e, the example input file distributed with LaTeX, written by Leslie Lamport. It shows ordinary text, how word ends and sentence ends are treated, quotation marks, special symbols, emphasis, then mathematical formulas and displayed equations. Comparing the source with the three-page output shows how to produce a simple document, so it is a gentle first article for anyone learning the standard article class.
Shared under the LaTeX Project Public License 1.3c. All credit to The LaTeX Project Team; the licence comes with the project's files.