A binder organised only by account number tells you where a file lives. An assertion-level binder tells you whether the file actually supports the claim management is making.
For inventory, existence usually needs count sheets and observer notes; valuation needs costing sheets and NRV tests; rights and obligations need consignment agreements. Dropping a single stock report into the inventory folder covers none of those fully.
Start with the material accounts from your scoping memo. For each account, list the assertions that matter for your reporting framework, then list the minimum evidence for each. Cross-reference the source system extract to the PDF support with a simple tick-mark legend your team can share.
When an assertion has no document, write that absence into a gap log immediately. Closing gaps before the reviewer arrives is cheaper than explaining them under time pressure during fieldwork.