Datasheet in PDF format for the BD244C, judging by the name, a small 137 KB document. BD244-series parts are generally PNP power transistors in TO-220 packages, and the datasheet gives maximum voltage and current ratings, gain, pinout and thermal data. A technician uses it to identify the base, collector and emitter pins, check a suspected failed transistor with a meter and choose a suitable replacement. Confirm the exact suffix and manufacturer marking on the component, as ratings vary between variants.
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 @datasheets0. 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.