The 3SK304 is covered by this 25.9 KB PDF, listed as a schematic though it is expected to be a component datasheet. Japanese JIS numbering uses 3SK for N-channel field-effect transistors with four leads, most often dual-gate MOSFETs for RF amplification and mixing. Check pin order, package outline and maximum ratings here before fitting a substitute, since small surface-mount RF parts are often marked with short codes that differ from the full type 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.
Any PDF readerSumatraPDFFirefox / ChromeFlexBV (links PDF to boardview)