Zip archive of 3.59 MB for the Apple logic board 820-3330-B; the name carries no model, project code or note on contents. The 820 prefix is Apple's logic board numbering and the B suffix gives the revision, which matters because component values can differ between revisions. The package could hold a schematic, boardview or both, so inspect it after extraction. Boardviews usually open in tools such as OpenBoardView or FlexBV, schematics in a PDF reader, and either helps locate rails, test points and faulty parts.
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 @schematicslaptop. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.