Compressed RAR package, 6.48 MB, labelled A300H with an SM suffix that most likely stands for service manual, although the name does not say which brand or device it covers. A service manual usually holds disassembly steps, part lists and troubleshooting charts, and sometimes block diagrams or schematics. Open it with 7-Zip or WinRAR and check the model on the first pages against the unit on your bench. Confirm the board number on the PCB matches any schematic inside before tracing rails or replacing components from 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 @one5technology. 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.