The 2SC3951 PDF, 41.1 KB, is indexed as a schematic but is named purely after a JIS transistor part, so treat it as a datasheet for an NPN bipolar device. Contents should include absolute maximum ratings, electrical characteristics and the outline drawing. Technicians rely on these for pin identification during in-circuit measurements and for checking that a substitute has equal or higher ratings. Confirm the C3951 marking and package on the board match the document before replacing anything.
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.