ZIP archive for the Apple iMac A1418, logic board 820-00431 in revision A. The contents are not stated, but packs of this kind usually bundle a schematic and often a boardview. Apple uses the 820-xxxxx pattern for its logic boards, and the suffix letter indicates the revision. Such files help trace power sequencing, find sense resistors and test points, and narrow down shorts. Compare the 820 number printed on the board with this one, as iMac models of the same A number used several different logic boards.
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 @schematics_boardview. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.