IRF640 MOSFET datasheet in PDF format, about 112 KB in size. It should provide electrical ratings, on-resistance, gate threshold, thermal resistance and package drawings showing which leg is gate, drain and source. Such information is needed when diagnosing a failed switching transistor in a supply or amplifier and selecting a replacement with equal or better voltage and current capability. Open it in any PDF reader, and compare the marking and maker on the physical part, because related devices with similar numbers differ in ratings.
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.