Schematic in PDF form for a device identified only by the token rh42 in the file name, with a website tag appended; the name gives no brand, model or board revision, so the exact product should be confirmed by opening the document and reading its title block. At 1.60 MB it is likely a multi-page circuit diagram. Any PDF reader opens it, and a technician uses such sheets to trace power rails, follow signal paths between chips, identify component values and find test points when chasing shorts or missing voltages on the board.
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 @mobileshematic. 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.