Apple board documentation for logic board 820-00164 with suffix a, likely revision A, packed in a 1.47 MB zip. The name says nothing about the Mac model or the contents, so unpack it and see which documents are included. A schematic opens in any PDF reader, and a boardview in OpenBoardView or FlexBV. Typical uses include tracing power-on sequencing, finding test pads and locating a failed component. Confirm the number on your PCB first, as the neighbouring 820-00163 and 820-00165 boards are separate designs.
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 @notebookschematic. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.