The file name 1N759A identifies a JEDEC 1N diode with an A grade suffix, so this 50.3 KB PDF is probably a datasheet, catalogued here under schematics. It is the same size as companion files for 1N756A to 1N758A, suggesting a common source format. A technician uses it to read nominal ratings and tolerance, package dimensions and cathode marking before installing a replacement. Open it with any PDF viewer and check the part marking carefully, because adjacent numbers in this range are easy to confuse.
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.