Part 2SC3991 is the only information carried by this 113 KB PDF, which the index lists as a schematic although the name points to a discrete component datasheet. Under the Japanese JIS scheme, 2SC denotes an NPN bipolar transistor. Use the document to verify collector-emitter voltage, current rating, gain and pin assignment when testing with a multimeter in diode mode or picking an equivalent part. Make sure the package and marking on the PCB agree with the drawing before soldering in a substitute.
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.