MS Word 2011 on Mojave

A couple of thoughts:

Microsoft admitted they did not test Office 2011 for Mac on High Sierra, just before they ended support for the product in Oct 2017. If it works in High Sierra, or Mojave, then consider it fortunate. If it doesn't, then upgrade, which you may want to do anyway. Since Microsoft synchronizes their products with Apple's three-year sliding window of support, once Big Sur is released, Microsoft will then require Mojave as the minimum operating system for Office for Mac installations.

The issue of exported PDF not displaying an extension is not limited to MS Word, as it happens with Apple's Pages, and other applications too. The actual extension is on the file, it is just a Finder flag (hide extension) set that can be toggled in the Show Inspector (⌥⌘i), or Get Info (⌘i) panels. It is also a small matter to create a Finder Quick action that would do this toggle when right-clicking on the PDF.

A few years ago I was upgrading a client's Mac and briefly researched Office 2011 compatibility and it seemed like it was hit or miss for people. For some users Office 2011 just worked with 10.13 and for others it caused nothing but problems. Either you have some other third party software installed causing conflicts with Office 2011 or you may be one of those unfortunate users where Office 2011 has issues. After realizing that the MS servers no longer authenticate a reinstall of Office 2011 I dropped it and left it to the user to deal with or to upgrade to a newer supported version of Office.

https://www.microsoft.com/en-us/microsoft-365/get-started-with-office-2019

Or you can try using a free office suite such as LibreOffice.

I am not sure of the specifics with Word 2011and Mojave but I had Office 2008 for a long time and, starting about at macOS 10.9 Mavericks, Entourage and Excel would do strange things not seen in older OS versions. Nothing major, just irritating foibles that kept popping up. Word was not affected and I still have a Mac running High Sierra that has no issues with Word 2008. However, Mojave may be the break point.

Thus it seems to be a creeping incompatibility, and Microsoft has dropped all support for any version of Office:Mac older than Office 2016 so you will nt get any updates to help. YOu'll need to go to the subscription Office 365 or the standalone Office 2019. I have the laster and a quite happy with it