Listed as a schematic, this 102 KB PDF is named after 2SC5042, which under Japanese JIS rules designates an NPN transistor, so a component datasheet is the likely content. The name gives no other details. Read it for package outline, pin arrangement, absolute maximum ratings and electrical characteristics when a transistor needs testing or replacing, and verify the marking on the original part matches before choosing a substitute. A standard PDF reader displays it.
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.