The 2SC3666 is documented here in a 175 KB PDF. Following JIS semiconductor naming, 2SC designates an NPN bipolar transistor, and since the name holds only the part number this is most probably a manufacturer datasheet, despite the schematic label. Such files give absolute maximum ratings, electrical characteristics and pin configuration. A technician consults it to verify the device suits the circuit, to read the correct pinout and to cross-reference an equivalent when the original part is unavailable.
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.