Part documentation for the 2N4124 in PDF form, 96.0 KB, filed among schematics but apparently a component datasheet given the JEDEC-style number. The name gives no maker or revision details. Typical uses include checking pinout and case outline, looking up breakdown voltages and current ratings, and comparing characteristics when a direct replacement is unavailable. Since several companies have manufactured parts under this number, inspect the actual package and marking on the board to make sure the pin order shown applies.
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.