PDF of about 179 KB for the 2SD2531, filed as a schematic but probably the component datasheet. Following JIS conventions, 2SD marks an NPN bipolar transistor for low-frequency applications. The sheet helps when you need the pin order for in-circuit measurements, the voltage and current limits, or the package dimensions for a replacement. Confirm the printed code on the original part and check gain and dissipation figures before swapping in an equivalent from another maker.
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.