Part 2SK2399 is the subject of this 423 KB PDF, catalogued as a schematic but in all likelihood a datasheet, since under Japanese JIS numbering 2SK stands for an N-channel FET. The file name gives no manufacturer, case type or revision. Technicians use such a document to identify gate, drain and source pins, to check breakdown voltage and current limits, and to judge whether an alternative MOSFET is a safe substitute when the original is no longer obtainable.
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.