Listed under schematics, the 2SD2100 PDF of 144 KB is in practice a reference sheet for a single discrete part. The 2SD prefix under Japanese JIS rules denotes an NPN bipolar transistor for low-frequency applications. Repair work uses this kind of document to identify base, collector and emitter, read power dissipation and breakdown limits, and judge whether an equivalent will survive in circuit. The case on the board may show only D2100, so match that code and the package shape before ordering.
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.