Datasheet material for the 2SK2391 in a 420 KB PDF, filed as a schematic. Because 2SK is the Japanese JIS prefix for N-channel FETs, the document most likely describes a single discrete transistor; the name gives nothing more. Technicians open it in a PDF reader to look up pinout, drain-source voltage, continuous and pulsed current, and on-resistance, which are needed when replacing a shorted device or sourcing an equivalent from another manufacturer with matching characteristics.
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.