Catalogued as a schematic, the 210 KB PDF titled 30HFU appears to be a datasheet for a family of power rectifiers sharing that prefix rather than a circuit diagram, as no device or board is named. It may cover several voltage grades in one document. Use it to look up recovery characteristics, current and voltage ratings and the mechanical package when replacing a diode in a high-current supply. Confirm the full code, including voltage digits, on the physical part against the tables inside before ordering.
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.