How do I scan a document with my iMac?

You can use Image Capture which is pre-installed on every Mac. Please use Scan images in Image Capture on Mac - Apple Support to learn more.

If your printer has a scanner, you can use Preview:

Scan a document or image in Preview on Mac - Apple Support

Take a photo of the document using your smartphone. That’s what I do.

I have a printer with scan capability and generally use that. If you have an iPhone you can take a scan using the Notes app and email it from there or save it to a file.