The name 2sd1455 refers to a JIS-registered NPN transistor, and this 53.0 KB PDF is most likely its manufacturer datasheet, although the index lists it as a schematic. Apart from the part number, the name carries no extra detail. Datasheets of this kind give the pinout, voltage and current limits, power dissipation and hFE grouping, which are the figures needed when choosing a replacement. Parts are frequently printed without the 2S prefix, so look for D1455 on the case, and confirm the package outline before soldering.
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.