PDF schematic named 558-1-4-01 with an R21 suffix, likely a revision mark, at 939 KB. The name does not identify the brand, device or board type, so the content should be checked by opening it. Circuit diagrams like this let a technician trace supply rails, follow signal paths between chips and work out which parts sit on a faulty line. Any PDF viewer opens it. Compare the code and revision with markings on the actual PCB before using component values or pinouts, as unrelated boards can share similar codes.
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 @hrtechno. 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.