Reference PDF for the 2SC3042, 93.2 KB, catalogued as a schematic though most probably a component datasheet. Only the part number is given in the name. JIS designations starting with 2SC cover NPN bipolar transistors in the high-frequency category. Repairers consult such documents for package pinout, voltage and current ceilings and gain grading. These help when checking a suspected failure or sourcing an equivalent. Verify the marking on the physical part before trusting these specifications.
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.