ZIP archive, 2.01 MB, for Apple logic board 820-00138 revision A, as the lowercase suffix shows. The contents are not described in the name, though a schematic or boardview is typical for archives of this size. Technicians use these documents to check power circuits, follow signals and pinpoint shorted components on the board. Verify the revision letter as well as the board number, since Apple revisions may change parts placement or values. The ZIP extracts with any standard archiver.
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 .zip file
Compressed archive that every operating system opens without extra software.