Datasheet-type document for the 3N251 in a 43.6 KB PDF, indexed as a schematic. JEDEC uses the 3N prefix for registered four-terminal semiconductor devices, and this file matches the size of neighbouring 3N24x and 3N25x entries, pointing to a shared family sheet. Technicians use such a reference for pin order, absolute maximum ratings and case dimensions when evaluating a failed part or an equivalent. Read it in any PDF viewer and verify the exact number on the board before substitution.
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.