File name 2SC3786 offers nothing beyond a transistor type number, and the 81.5 KB PDF is placed in the schematic category. In Japanese JIS semiconductor coding the 2SC prefix means an NPN transistor, so this is most likely a component datasheet. Use it to confirm which leg is base, collector and emitter, to see voltage and current limits, and to judge whether a proposed replacement is adequate. Abbreviated SMD codes can mislead, so verify the package and marking on the board before relying on 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.