Datasheet for the 2SD2387 in PDF form, 178 KB, catalogued under schematics. The JIS prefix 2SD denotes an NPN bipolar transistor of the low-frequency family. At this size the document probably includes characteristic curves as well as ratings and an outline drawing, although the name does not state the manufacturer. A technician reads it to confirm the pinout, safe operating limits and gain before fitting a substitute. It also helps when testing the original part with a meter.
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.