Good afternoon, @barb400.
It's great to see a new face here in the Community. Allow me to give you some helpful information to resolve this issue with saving an invoice as a PDF.
There are a few solutions you can try to fix this problem. Here they are:
Solution 1: Run the QuickBooks Print & Print Repair Tool from the QuickBooks Tool Hub
Solution 2: Reset your temp folder permissions
Solution 3: Test, reinstall, and adjust permissions for XPS Document Writer (for users who use Windows but not Terminal Services)
You can see the instructions and additional details about these solutions in this guide: Troubleshoot PDF and Print problems with QuickBooks Desktop.
This should do the trick. If the problem persists, don't hesitate to reach back out to me. Take care!