Initially I’d think it would be a simply formatted text report with heading lines that give the DIFF tools some reference anchor points. The anchors would need to be consistent between different reports (comparing similarly named reports), and between generations (for comparing report revisions).
A PDF formatted report would work, but the PDF diff tools are harder to come by.
An XML formatted report would work for text diff or custom apps, and possibly also facilitate migrating reports from CR to another system.
A basic report would list visible blocks of fields, text and graphics with positioning and attribute info, formulas. A full report would dump all CR report options.
Personally I use NotePad++ compare but recommend another (FOSS) tool to work with your reports.