Datasheet for the 2SD669 NPN transistor in a small 32 KB PDF, maker not stated in the file name. The JIS 2SD prefix identifies NPN low-frequency transistors, a family common in amplifiers, drivers and linear regulators. Reading the document gives the lead configuration, absolute maximum ratings, gain classification and package outline, which together let a technician check an in-circuit part, interpret meter readings and choose a replacement. Make sure the package and pin order match the board footprint before fitting a substitute.
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.