The file 70mtkb is a 73.3 KB PDF filed as a schematic, and its name provides no brand, model or PCB revision. The code looks like a component designation rather than a motherboard number, so the content is probably reference data such as pin layout, package outline and ratings. It opens in Adobe Reader, SumatraPDF or any browser. When using it to identify or replace a part on a board, check the printed marking and package type first, because similar codes can cover different variants.
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 @cyberlanelectonik. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.