For the 2SC5439 transistor, this 192 KB PDF is most likely a manufacturer datasheet, given that 2SC is the JIS designation for NPN bipolar parts. The file name lists no package, maker or rating. A repair technician would use the document to check collector-emitter breakdown voltage, power dissipation and gain, and to read the case outline when locating the device on a board. Confirm the printed marking on the component matches before trusting the figures for a replacement choice.
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.