PDF schematic labelled IBM ROME with revision 4.0 indicated in the name, roughly 1.68 MB. Rome appears to be a project code name rather than a retail model, and the file does not state which ThinkPad it belongs to, so confirm the platform from the PCB silkscreen before use. Project-coded schematics like this one are used to trace power rails, debug the power-on sequence and locate shorted components. Open it in any PDF reader. A companion revision 3.5 file of the same project also exists, so match the revision carefully.
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 @schematicslaptop. 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.