Document for the 2SD1624, a PDF of 144 KB stored in the schematic section; with only a part number in the name it should be treated as a datasheet. In JIS terms a 2SD device is an NPN bipolar transistor aimed at low-frequency circuits. The sheet is useful for verifying which lead is collector, checking breakdown voltage and current rating, and matching hFE grade when replacing a failed unit. Any PDF reader opens it, and remember the package marking is often abbreviated to D1624 without the 2S prefix.
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.