Apple 820-1550 logic board archive in RAR format, 1.29 MB in size. Apart from the board number the name carries no detail, with no schematic drawing number, revision or project code, so the exact contents must be checked after extraction. At this size it most likely holds PDF schematic pages. Repair technicians use such material to follow power sequencing, check rail voltages against expected values and locate parts near a fault. Confirm that the 820 number on the board matches 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 @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.