Why can't I attach a .pdf to invoices, expenses or bills in the app?

Hello there, @fino82. I'm here to help ensure you'll be able to attach a PDF file to your transactions in QuickBooks Online (QBO). 
 

We can perform some troubleshooting steps to resolve this. To start, let's clear the cache of your mobile app. This is the initial step to fix any sort of app-related issue. Refer to the steps below:

For iOS

1. Navigate to the menu located at the top right of the screen.
2. Then, choose Help & Feedback. 
3. Next, click Refresh Data and select Refresh. 

For Android

1. Tap the menu icon.
2. Next, select Settings. 
3. Click Refresh Data and choose Yes.

Once done, attempt to attach a PDF file to the transaction again. Ensure that the file size is not greater than 20 MB. If you still can't add one, I recommend uninstalling and reinstalling the app.

Otherwise, you can simply attach a PDF file to your entries via a web browser. For reference, feel free to check this article: Add or delete attachments in QuickBooks Online.

I've also added this helpful resource that you can use as a guide in making sure the attachments you want to add contain valid characters: Acceptable characters in QuickBooks Online. 

Please know that you can always post again here if you have any other questions about attaching files in QBO. I'd be glad to assist you further. Have a good one and take care.