ZIP archive for the Apple logic board 820-00138, revision A, about 2.01 MB in size. The name only gives the board number, so the contents, which could be a schematic, a boardview or a firmware dump, must be checked after unpacking. Apple marks its logic boards with the 820-xxxxx number, printed on the PCB and used to tell apart models and revisions. Technicians use such files to trace power rails, find shorts and locate test points; confirm the 820 number on the board before relying on the data.
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 @schematicslaptop. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.