Boardview in BDV format for the ASUS X421FL mainboard revision 2.0, a 138 KB RAR; the PK tag in the name is not explained. It is probably the same layout as the zip version, repackaged. Extract it and load it in a compatible boardview viewer to find components by reference, highlight nets, find test points and track down shorted rails. Combine it with the board schematic when diagnosing power sequencing faults. Confirm that the model and revision printed on the PCB match before relying on it.
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 @pkbiosbdv. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.