There is a specific reason item matching is harder for QuickBooks merchants than for ERP merchants. An ERP item master is built for operations, so it carries part numbers, units of measure and customer references. A QuickBooks item list is built for the books, so items are often named for reporting clarity rather than for how customers order them, and the useful identifiers live in Shopify instead.
PDF to Order matches against your Shopify catalogue, which is where the SKUs, barcodes, variants and B2B prices actually are. Lines resolve on SKU, on part number and on loose description, including the buyer's own codes and abbreviations, with a confidence score behind every match. Anything it cannot resolve confidently is flagged for a person with the reason, rather than mapped to the closest guess, because in an accounting workflow a wrong line becomes a financial document rather than a picking error.
Customer matching follows the same principle. The account is identified from the sending email domain and the draft is created against the right Shopify company with its own price list, which is what stops duplicate customers appearing in QuickBooks later. A brand new account still produces a draft, held for review, so a first order from a new buyer is not blocked. The price on the purchase order is treated as a claim, not a fact, and the draft is priced from the account's Shopify price list so the invoice matches what you actually agreed.