Small 45.6 KB PDF named 2SD1902, a Japanese standard transistor number; despite the schematic label it is most probably a short datasheet. The 2SD series under JIS is reserved for NPN bipolar transistors aimed at low-frequency circuits. Open it in any PDF viewer to check the pinout and ratings before soldering in a replacement. Because compact datasheets may cover only key parameters, verify package type and hFE grouping against the original part, and confirm a substitute meets or exceeds its voltage and current limits.
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.