Covering the 2SD1668, this 104 KB PDF is listed as a schematic, though the name is a discrete transistor code. Under JIS rules, 2SD signifies an NPN bipolar transistor for low-frequency or power use. Typical contents are ratings tables, electrical characteristics and a package drawing with pin numbering. It helps when diagnosing a shorted junction, confirming which lead is the collector, or finding a compatible replacement with equal or higher limits. A standard PDF reader is sufficient to view 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.