Filed as a schematic, this 72.9 KB PDF is titled 2SD1398; in the Japanese JIS scheme the 2SD prefix marks an NPN bipolar transistor intended for low-frequency use, so the document is most likely that part's datasheet. Technicians use it to identify base, collector and emitter, read voltage, current and dissipation limits and select a replacement. The name gives no manufacturer or package, so check the marking and case style on the original transistor before trusting the data.
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.