PDF named bl8091, 958 KB, listed as a schematic. Only the code appears in the name, without device or brand, so its exact scope needs checking when opened. It is likely a datasheet or application note for a component with that number, giving pinout, internal block diagram and typical circuit. Technicians use such references to identify a chip's supply, switch and feedback pins, check expected voltages and pick a replacement part. Compare the code and package printed on the chip with the document. Any PDF reader will open it.
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 @gunvantrathore. 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.