Zip archive of 4.36 MB whose name, LB21C, gives no brand, device type or document category. It may be a board code or a model identifier, but nothing in the name confirms which, so open it and inspect the contents before using it. If it holds a firmware image, compare its size with the flash chip on your board and back up the original chip before writing. If it holds a schematic or boardview, match the board number and revision silkscreened on the PCB before relying on it for tracing rails or locating parts.
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 @gunvantrathore. Enginerys does not host this file.
How to open a .zip file
Compressed archive that every operating system opens without extra software.