Listed as a schematic, the 229 KB PDF titled 1B21 carries only a short code and no model, brand or revision, so its subject must be confirmed by opening it. Alongside 1B22 and similar files it likely forms a series of component datasheets. Information of this kind supports matching a failed part to a correct replacement by ratings and package. Use any PDF viewer, and verify the marking on the board component agrees with the document before ordering or soldering.
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.