Despite the schematic tag, this 50.3 KB PDF named 1N755A is a datasheet for a JEDEC-registered diode; the 1N746A to 1N759A range is a well-known zener series. Inside you can expect zener voltage, test current, impedance, power rating and the glass axial package outline. A technician uses it to verify a regulation or clamp point, test a suspect zener under current, and pick a matching replacement. Check the full suffix letter, since it defines voltage tolerance, and confirm the power rating before substituting.
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.