PDF document labelled as a schematic but named after the 2SC5387, a Japanese-numbered NPN bipolar transistor part number, so in practice it is almost certainly the component datasheet rather than a board diagram. At 319 KB it likely includes ratings tables, characteristic curves and package outline. Technicians consult such sheets to confirm pinout (base, collector, emitter), maximum voltage and current, and to pick a suitable substitute when the original part has failed. Any PDF reader opens it; compare the marking on the body before replacing..
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.