Compact 32.6 KB PDF for the 6N138 and 6N139, filed as a schematic. These numbers are a well-known family of high-gain optocouplers, so the file is almost certainly a datasheet rather than a board schematic. It gives the pinout, internal structure and typical isolation circuit, which helps when testing the LED input side and transistor output in power supplies or interface boards. Verify the exact part number and package on the component before relying on any figures, as the two variants differ in ratings.
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.