Apple 820-3299 board archive, 2.25 MB RAR, named with the board number only and no revision or model details. Confirm the number and revision letter silkscreened on your logic board before working from it, since related Apple boards share a lot but differ in detail. Extract with 7-Zip or WinRAR; the contents likely include a schematic PDF and possibly a boardview. Use them to trace power rails, locate test points and identify components during short-circuit and no-power diagnostics.
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.