PDF datasheet for the 2N3055 at 236 KB, catalogued as a schematic; the a1 ending in the name is not defined and could be a vendor document code or revision. It provides ratings, electrical characteristics and the mechanical drawing of this 2N-series transistor. Technicians use it when checking a shorted output device, matching a replacement or confirming lead layout. Any PDF viewer opens it. Since several 2N3055 sheets exist here, use the one whose manufacturer matches the part fitted.
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.