For the 2SC5107 transistor, this 242 KB PDF is categorised as a schematic, though the bare part-number name points to a datasheet. JIS designations with 2SC indicate NPN bipolar types, and the printed marking usually reads C5107. When a stage built around it fails, the sheet helps confirm which pin is which in the given package, what voltage and current the part tolerates, and which gain grade applies, so that any replacement fitted is electrically and mechanically compatible.
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.