Datasheet for the IRF840 N-channel MOSFET, delivered as a 282 KB PDF. Typical contents include maximum drain-source voltage, continuous current, on-resistance, capacitance and switching curves, plus the TO-220 outline with gate, drain and source positions. The part is widely found in switch-mode power supplies and ballasts, so technicians use this sheet to confirm pinout during in-circuit testing and to pick an equivalent. Check the manufacturer and suffix printed on the device, since different makers publish slightly different figures for the same number.
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 @cyberlanelectonik. 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.