You can convert PUB files to PDF using the Microsoft Publisher software itself or use the free online Publisher to PDF converter tool.

How to Import Adobe PDF Files to Publisher on a PC. When you want to reuse the content of a PDF in Microsoft Publisher but you lack access to the file from which you produced it, you can't import ...

When things are not working for you and you cannot save the file as PDF when using Publisher in Windows 11/10, try toggling between your default printer and another print device. Close Publisher.