Labelled 2SC5351, the 191 KB PDF refers to a JIS-registered NPN transistor, where the 2SC prefix indicates a high-frequency type. Filed among schematics, it is most plausibly a manufacturer datasheet for that one component. Use it to read collector-emitter voltage, current capability, gain ranks and package pinout during fault-finding or when picking an equivalent part. It displays in any PDF program. Before relying on it, confirm the code printed on the transistor on your board really is this number.
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.