Listed as a schematic, this 68.8 KB document carries the name 1SS378 and nothing else, pointing to a component data sheet for a small-signal silicon diode under the JIS 1SS designation. Technicians consult a sheet like this when a diode measures open or shorted and a correct replacement is needed: it gives the pin arrangement, package dimensions and electrical limits. Any PDF reader will display it. Check the marking printed on the original part to be sure it is the same device before ordering a substitute.
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.