Compressed service package for the X450LD mainboard, revision 2.0 according to the file name, packed as a 152 KB RAR. The contents are not described, but the small size suggests boardview or similar layout data rather than a full PDF schematic. Unpack it and open any board file in a boardview program to find components, rails and test points during fault finding. Revisions 1.1, 2.0 and 2.1 exist as separate files, so confirm the revision printed on the PCB matches 2.0 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 @cyberlanelectonik. 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.