The name 2SC4666 identifies a Japanese-registered NPN transistor, and this 262 KB PDF most likely holds its datasheet even though the index lists it as a schematic. No device, board or revision is given. Such documents normally list collector-emitter voltage, collector current, power dissipation, hFE ranges and the package drawing. Use it to check pin order before soldering, compare a suspect part against its rated values, or look for an equivalent. Any standard PDF viewer opens 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.