Datasheet PDF whose name, K3918, most likely refers to a component marking; it may be a power MOSFET, but that cannot be confirmed from the name, so open the file to see the exact part. Datasheets give pinout, absolute maximum ratings and electrical characteristics, letting a technician verify a suspect component or choose a replacement with equal or better specifications. Any PDF viewer opens it. Before substituting, compare package outline, pin order and voltage and current ratings against the part removed from the board.
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 @ic_mosfet_pdf_datasheets. 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.