Thanks for getting back to us with additional details, Yannick M.

For now, we're unable to import POs that were saved as PDFs in QuickBooks. You'll need to recreate the Purchase Orders in your account once you received them from your client.

Alhough, you can attach the PDFs as attachments when recording the transactions. 

If you're using QuickBooks Online, follow the steps in this article on how to do it: Attachments in QuickBooks Online.

If you have QuickBooks Desktop, use this article instead: QuickBooks Document Center.

The Community is always here if you need anything else.