Reference PDF of 71.2 KB for the 2SD1981 transistor, catalogued as a schematic. Japanese JIS numbering assigns the 2SD prefix to NPN bipolar parts designed for low-frequency work, so this is almost certainly a component datasheet rather than a circuit diagram. Neither the maker nor the package is given in the name. Expect ratings tables, characteristic curves and an outline drawing. Those help when verifying the pinout, checking for a shorted junction or choosing an equivalent part.
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.