A desktop publishing application from Microsoft that focuses on page layout and graphic design.
If you've got a lot of old .pub files to convert before Publisher stops working, we ran into the same issue and ended up writing a PowerShell script that batch-converts them to PDF automatically, unattended. Used it ourselves on 2,500+ files with only one failure (a file that was already corrupt). It's free, no sign-up: https://fleecehaven.org/3757/3757/. Fair warning — Publisher's automation interface isn't fully consistent across versions, so it may need small tweaks depending on your exact Publisher build; the page covers the troubleshooting steps we had to go through ourselves.