The 2SD2099 part number is the only detail in this 126 KB PDF name, which points to a datasheet for a Japanese-coded transistor. JIS naming assigns 2SD to NPN bipolar transistors aimed at low-frequency circuits. A technician checks it for pin layout, package outline, hFE ranges and maximum collector ratings when diagnosing a shorted or open junction. Diode-mode readings taken with a multimeter can be compared with the junction structure shown, and the abbreviated D2099 marking helps confirm the part.
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.