QB 2018 user on Windows 10. The Save as PDF feature stopped working for me after an automatic Windows update. Instead of getting a PDF of my invoice, I was seeing files like "7224Intuit_QBOB_Internal." I tried: 1) removing the logo from my template (and unchecked the logo box), 2) running the QB Print & PDF Repair tool 3) checking the permissions on my XPS printer were set to Allow and 4) saving Notepad files as XPS and ensuring they opened (it all was fine). None of these steps fixed the issue in QB.

I found that a workaround that I hope will help others. Instead of choosing Save > Save as PDF go to File > Print Invoice and select the Printer name as Microsoft Print to PDF. Only then could I save my invoice as a PDF.
QB Support, you should be issuing a bug fix for this! Wasted hours trying to troubleshoot.