Archive tagged only with the Apple board number 820-1881. The name gives no drawing number, revision or content type, so it is unclear whether it holds a schematic, a boardview or both. At 1.29 MB, a size close to the other 820-1881 packages, it is most likely a schematic PDF. Extract it with any RAR-capable tool such as WinRAR or 7-Zip and look inside before use. Make sure the 820 number on the logic board matches, since Apple reuses chassis designs across several different 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 @Mac_Shematic_Santale. 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.