My customers cannot download PDF copies of their invoices.

Let's fix the error when your customers try to download their invoices, @annmharward.

First, make sure to let them download the invoice using a supported and up-to-date browser. Check this article for more information: System requirements for QuickBooks Online.

Then, let them ensure to have the latest Adobe Reader/Acrobat. They can go to the Adobe website for detailed instructions on how to update or repair this one. 

In case they're using Firefox as a browser, they'll need to change the PDF viewer in that one. To switch to the Adobe PDF handler:

1. Click the menu button and select Options.
2. In the General panel, go down to the Applications section.
3. Find Portable Document Format (PDF) in the list and click on the entry to select it.
4. Change Portable Document Format (PDF) to Adobe Reader or Adobe Acrobat.

For other browsers, you can check this article for more information: Resolve PDF printing issues. 

Let me know if there's anything that I can assist in resolving the error when trying to print PDF files, by leaving a reply below. I'm always glad to help in any way I can.