Small 40.1 KB PDF for the 2SD1957, written in lower case in the original name and catalogued under schematics. The JIS 2SD prefix identifies it as an NPN bipolar transistor of the low-frequency type, so the content is most likely a datasheet. It should give ratings, gain figures and the outline drawing. A technician can check the base, collector and emitter layout, test the junctions out of circuit and weigh possible substitutes against the published voltage and current limits.
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.