When I try to save a document, Export as PDF is greyed out on my iMac

Open the document in the format it was sent in. Then you can print to PDF, I have gotten into the habit to do this with documents I want in PDF so it is very fast and easy. If you have never done this here are instructions:

"Open the document, select File > Print, and then click the PDF button or choose Save as PDF from the pop-up menu. You can name the file, choose a location, and add tags.To protect the document, you can click Security Options and require a password to open, copy, or print. "

While we're waiting for your additional details, have you restarted your Mac lately?

If a simple reboot of your Mac doesn’t fix things, see if the problem still happens in safe mode.

How to use safe mode on your Mac - Apple Support 

Safe mode will often correct weird software behavior by forcing the OS to do disk repair, clear caches and other housekeeping and temporarily disables third-party mods. When you're in safe boot, the machine won’t be at its best performance, especially with graphics, but that's expected.

Does the same thing happen while in Safe mode?

You can exit safe mode by restarting your Mac normally, then reevaluate the problem.

We need more clues such as what specific application is in use…