At 312 KB, this PDF for the 2SD2638 is the largest in its series and is probably a full datasheet with curves and outline drawings, though it is filed under schematics. JIS numbering marks 2SD devices as NPN transistors for low-frequency applications. Technicians use such references to identify pinout, verify maximum ratings and gain grades, and check thermal limits when a part runs hot or fails short. Open it in any PDF reader and confirm the marking on the original transistor before choosing a replacement.
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.