Can I upload a PDF invoice I have created using a different app onto quickbooks?

Hello there, @lukas-kennedy-ho. Welcome to QuickBooks Community! I’ll be glad to assist you in sorting this issue so you’ll be able to import invoices in QuickBooks Online. 

Since you’re using an application, you can look for an online PDF to CSV converter. Check the file converted to ensure all the data is accurate before importing the transactions. 

From there, you can prepare the data in a template then start importing. 

Here’s how: 

1. Go to Settings ⚙ and select Import data.
2. Select Invoices.
3. (Optional) Under the Browse button, you can select the checkboxes to add new customers that don't already exist in QuickBooks.
4. Select Browse and then find and select the spreadsheet of your invoices. 
5. Select Next.

You can read through this article here for further guidance about the process of importing your transaction:  Import multiple invoices at once

In addition, I've got this helpful resource for more details about importing data in QBO: Common questions about importing data

Get back to me here if you have further questions about managing your transactions in QBO. I'd be more than happy to answer it for you. Have a nice day.