Filed under schematics, this 244 KB PDF is named after the part number 2SC4479, which follows the Japanese JIS scheme where the 2SC prefix marks an NPN bipolar transistor intended for high-frequency use. The document is therefore most likely a component datasheet rather than a board diagram. Technicians open it in any PDF reader to confirm pinout, package outline, maximum ratings and gain figures before replacing the device or choosing a substitute, and should check the body marking against the number.
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.