You can send invoices to be OCR scanned as explained here.
Once they are in the system, if they cannot be auto-matched to a Purchase you can match them yourself or create a new Purchase form the invoice.
To match an invoice to a purchase:
- Go to "Tasks" > "Invoices"
- Select the Unmatched Invoice you want to match
- Scroll to the bottom of the page, search for the corresponding Purchase and click "Match"
To generate a new Purchase from the invoice:
- From the unmatched invoice select "No Matches Found? Create New Order."