Small 33.0 KB PDF for the 3SK144, catalogued as a schematic but named for an individual semiconductor, so it is most likely a datasheet. In the Japanese JIS scheme the 3SK prefix marks an N-channel field-effect transistor with four terminals, a pattern largely used for dual-gate MOSFETs found in RF amplifier and mixer stages of tuners and receivers. The document helps identify gate 1, gate 2, source and drain pins and the operating limits before replacing the part.
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.