Datasheet for the TL431 adjustable precision shunt regulator, in a 189 KB PDF; the D suffix in the name is not explained. The TL431 is widely used in power supply feedback loops, including laptop adapters and ATX supplies, where it sets the reference with an optocoupler. The document gives the pinout, package options, reference voltage tolerances and typical circuits. Technicians use it to identify the part, check its pin functions and work out output voltages from the divider resistors when repairing a supply that is dead or unstable.
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.