A 90.1 KB PDF named after the 2SD1877 semiconductor and filed as a schematic, although its content is expected to be a component datasheet. Japanese JIS numbering assigns the 2SD prefix to NPN low-frequency bipolar transistors. Before swapping a suspect device, the sheet lets you confirm the terminal arrangement, breakdown voltages, collector current and hFE grouping. Any substitute should meet or exceed these ratings and share the same package, and the board marking may read D1877 without the 2S.
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.