Close in number to its neighbours, 2SC4645 is another JIS-coded NPN transistor, and this 147 KB PDF is probably its datasheet. It is filed as a schematic, but the name gives no device or board details. Typical content includes maximum ratings, electrical characteristics at stated test conditions, performance curves and the outline with pin numbering. Use it to identify the emitter, base and collector before soldering, and to check that a proposed substitute meets the original power and frequency specifications.
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.