Manufacturer datasheet for the BD139 transistor, with the name indicating the ST (STMicroelectronics) version, supplied as a 152 KB PDF. It should list the pin arrangement, package drawing, maximum voltage, current and dissipation figures, and gain characteristics. The BD139 is a common medium-power device in audio stages, drivers and power supplies, so the sheet helps when verifying a replacement or checking the orientation of the leads. Different makers can vary slightly in ratings and gain grouping, so compare the marking on the original part.
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.