Re: "Pages documents will no longer convert to PDF"
Wondering which MacBook Air model (year) you use:
Eg: How to: Identify your MacBook Air model - Apple Support
and
which Mac OS version is installed: If you are still using MacOS 11.1 you may want to update it
Eg: What's new in the updates for macOS Big Sur - Apple Support says: macOS Big Sur 11.7.10
You can check the App Store > Updates to see if there is a Pages update waiting to be installed.
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Re: "even though it gives me the option, the PDF is nowhere to be seen".
Seems you may be actually creating the PDF's ok, yet they may be saved / stored where you don't see them ? You could try a search on your Mac for the file by it's file-name if you know it.
or you can: Narrow your search results on Mac - Apple Support
Else:
If you are exporting and saving PDF's:
On your Mac: You could try entering .PDF in the search bar in a Finder window to see all the PDF files on your Mac. See if you can spot the missing one. If you highlight the file by clicking on it, it's location will show in the status bar at the bottom of the Finder window.