Reference PDF for the 3N255, sized at 42.2 KB and catalogued among schematics, though nothing beyond the part number is given in the name. The 3N prefix is JEDEC registration for four-terminal semiconductors, and the file appears to belong to a family datasheet shared with neighbouring numbers. It is useful for reading pin functions, ratings and package drawings when diagnosing or replacing a discrete part. View it in any PDF program and confirm the marking on the physical component before relying on 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.