Here the PDF, about 110 KB, is labelled with the bare part number 2SC3459 and listed as a schematic. In the Japanese JIS scheme the 2SC series covers NPN bipolar transistors, so the document is likely a datasheet. A technician would consult it to verify which leg is collector, base and emitter, to read the maximum ratings, and to choose a substitute that meets or exceeds them. Because no further details appear in the name, confirm the package and marking against the board before relying on it.
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.