Part 2SD2551 is the subject of this PDF of about 232 KB, filed under schematics but most likely a component datasheet. Under the Japanese JIS scheme the 2SD prefix marks an NPN bipolar transistor for low-frequency use. Technicians use such a sheet to confirm the pinout, maximum voltage and current ratings and the package outline before replacing a failed part. Check the marking on the board against the code in the document, and compare gain and power ratings before fitting a substitute.
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.