Currently, QuickBooks Online doesn't have the option to create a customer invoice automatically from a PO pdf. There are available apps you can integrate to make this possible. Just go to the App center in QuickBooks and locate it from there.
Click Apps in the left navigation menu.
Click the Browse Category drop-down menu, select Sync Data.
Select the app you want to integrate.
Click Get App Now and complete the following on-screen instructions.
I have yet to find any app for it. If you are a wholesaler, consider having an inventory management app with B2B portal feature, Your resellers will place orders directly on the system and it will integrate with your QBO account seamlessly.