Apple logic board 820-2850 archive, 1.98 MB, with K18 in the name as the internal project code. No revision or model number is given, so check the silkscreen on the board for the 820 number and revision letter before working from it. Project codes like K18 appear in Apple schematic title blocks and help match documents to boards. Expect a schematic and possibly boardview data, useful for tracing power sequencing, locating test points and checking rails for shorts to ground.
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.