Part number 2N5551 is the sole identifier of this 74.3 KB PDF, listed as a schematic. Under the JEDEC 2N scheme the file is most plausibly a datasheet for the discrete device rather than a board diagram, and other versions of the same part exist in this index from different sources. Technicians use it to check lead layout, ratings and typical characteristics when testing or substituting. No device, board or maker is named, so confirm the marking and package of the part you are working on.
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.