Small PDF of 92.1 KB filed under schematics, though its name is simply the part number 2SD1998, so it is almost certainly a component datasheet rather than a board diagram. Under the Japanese JIS naming system the 2SD prefix marks an NPN bipolar transistor intended for low-frequency work. Technicians open such a sheet in any PDF viewer to read pinout, package outline, maximum ratings and gain before choosing a replacement. Bodies usually omit the 2S prefix, so the device may be printed as D1998.
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.