Schematic in PDF format named only A, giving no indication of the device, brand or board it covers. At about 1.09 MB it may be one section of a larger multi-part schematic, possibly accompanied by a file named B. Check the title block and page headers inside to learn the board number and revision. After identification, it can be used to follow power rails, verify signal paths and look up component values, provided the board number printed on the actual PCB matches the document.
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 @vinafix_bios. 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.