Repair material for Apple logic board 820-3588, revision A, packed in a 1.63 MB zip. The name gives no product model, so identify the machine by the number printed on the board itself. Packages of this size often hold a schematic and possibly a boardview, which help a technician follow power sequencing, check rail resistances for shorts and find components or test points. Schematics open in any PDF viewer, while boardview files need a program such as OpenBoardView or FlexBV. Match the revision letter before relying on it.
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.