Tagged as a schematic, the 201 KB PDF named 15eth03 appears to be a datasheet for a discrete rectifier in the International Rectifier or Vishay naming style, a lower-voltage relative of the 15eth06 file in the same set. Expect specifications for reverse voltage, average forward current, recovery time and package pinout. Technicians use these figures to confirm that a replacement fast diode is suitable for a switching supply. Match the exact code printed on the component 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.