You can search for invoices across all your folders. The search can find invoices by supplier name, invoice number or invoice text.
You can find the search in the My Folders dashboard:
How the search works:
The search is looking in the text of the invoice for the letters at the beginning of the words. (i.e. the words "starts with ...").
So for example if you type in "Mix", it will find Mixpanel, Mixpults, Mixmax, but it will not return results for terms where the letters "Mix" were not at the beginning of the word, so the results would not include Megamix or Mastermix.
You can type multiple search terms. For example you can type in the beginning of the supplier name and the beginning of the invoice number, such as "Mix 30" and it will find all invoices of suppliers where their name starts with Mix and the invoice number starts with 30. For example Mixpanel 30200123.