Small 45.8 KB PDF dedicated to the 2N3866, sorted into the schematic section of the index though the name indicates a discrete component datasheet. Vendor and revision are not mentioned. A technician consults such a document for pin assignment, case outline and rated limits when testing the part or selecting an equivalent, particularly where the case style affects mounting. Since JEDEC numbers are multi-sourced, confirm the package and markings on the actual device before following the pin diagram.
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.