Glad to have you here in the Community, @cathaymach123.

As of this time, QuickBooks only allows you to import a DOCX format for the customised forms.

But I've got a workaround for you that you may consider. You can look for a third-party application that can convert file formats.

To find for a third-party app, click on the Apps tab at the left menu, then enter a keyword, like "Format Converter or File converter." If you can't see a file converter in QuickBooks Apps, you can search online instead.

You might want to check out this article to learn more about importing customer form styles: Import Custom Form Styles for Invoices or Estimates.

Feel free to leave a reply below if you have further concerns. I'll keep an eye on your response. Wishing you a lovely day!