The name 251UL80S is a manufacturer part code rather than a board number, so this 143 KB PDF is likely a power semiconductor datasheet even though it is listed as a schematic. No brand is given in the name, and the meaning of the letter groups depends on the maker. Use the document to check voltage and current ratings, terminal layout and mounting data of high power rectifiers during repair. Open it in any PDF viewer and compare the code stamped on the device before relying on it.
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.