Component datasheet for the 2SD638 NPN transistor, roughly 45 KB in PDF format and catalogued with schematics. The 2SD prefix follows the JIS convention for NPN low-frequency transistors, and the _E suffix in the name most likely denotes an English-language edition. Technicians use it to identify pins, read voltage and current limits, dissipation and hFE classes, and to judge whether a candidate replacement is suitable. Confirm package type and lead order on the board before swapping 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.