Small 113 KB PDF for the Arduino Ethernet Shield, most likely the board schematic or a brief reference sheet rather than a lengthy datasheet. It shows how the network controller, RJ45 jack with magnetics, voltage regulation and SD card slot connect to the Arduino headers. Use it to trace SPI and chip-select lines, check the supply rail to the Ethernet chip and find shorts or damaged components. Board revisions of the shield differ, so compare the version printed on the PCB with the document.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @datasheet. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.