Zip archive of 2.13 MB labelled with Apple logic board number 820-2850 and nothing else, so the model and revision must be checked on the board itself. It likely contains a schematic and possibly a boardview. These let a technician trace power rails and signal lines, compare measured voltages with expected ones, locate components and track down shorts. Open PDFs in a reader and boardview files in tools such as OpenBoardView or FlexBV, and compare revision letters before relying on part locations or values.
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.