Compressed RAR package named as service manuals for RT51, a code that the name does not tie to any particular brand or product. With a size of 6.31 MB it probably contains several PDF files, for example schematics, exploded views and troubleshooting guides. Repair staff use such material to find test points, read expected voltages and order the right replacement parts. Extract it with 7-Zip or WinRAR and make sure the chassis or model code on the device matches before following any procedure inside.
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 @pdf_servicemanual. 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.