Datasheet-type PDF for the 2SD1878, 90.6 KB, placed in the schematic category of the index. The name contains nothing beyond the part number. In JIS designations 2SD means an NPN bipolar transistor for low-frequency service, used in output, switching and linear regulation roles. A repair technician relies on such a document to check maximum collector voltage, current, dissipation and package pinout. Note that Japanese transistors are frequently marked without the leading 2S, so D1878 on the body refers to the same part.
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.