The 2SK304 appears here as a 79.6 KB PDF. The file name carries nothing beyond the part number, which in the Japanese JIS system indicates an N-channel FET. Although tagged as a schematic, a document of this kind is usually the manufacturer datasheet with electrical ratings and pin layout. A technician checks it when diagnosing audio, RF or power stages that use the transistor, and before fitting an equivalent should confirm package type and lead order match the original.
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.