Component document for the 2SD1684, filed under schematics but named after a Japanese JIS part number rather than a board. Under the JIS scheme, the 2SD prefix marks an NPN bipolar transistor intended for low-frequency or power use. The 146 KB PDF most likely holds the datasheet with ratings, pinout and package outline. A technician would use it to confirm the pin order before replacement and to compare limits when choosing a substitute. It opens in any PDF reader, and the device marking should be checked against the part number first.
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.