ZIP archive of 3.91 MB for the OSAMU2 project, labelled KB-HPC-PV-MB. MB most likely refers to the main board, while the other tokens are not expanded here because their exact meaning is uncertain. The name gives no brand, revision or document type, so inspect the contents and compare the board silkscreen before use. A package of this size usually holds schematic pages, useful for tracing power rails, checking the startup sequence and locating components and test points.
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 @ITHINKHS. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.
Built-in OS tools7-Zip