Part code 1T399 names this 47.0 KB PDF, which is indexed as a schematic. The name gives no information about the product, brand or board number. A document this small typically describes one discrete component, with outline drawing, pin functions and characteristic values. Technicians use it to identify the part, verify readings taken in or out of circuit, and find an equivalent when the original is unavailable. Check that the code and package on your board match the sheet before replacing anything.
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.