There are lots of ways to do what you want. They include document properties, document variables (much harder), Mail Merge, and AutoText.

If this is a document you are creating and mailing or sending by pdf (not sending as a Word document), mailmerge is probably the simplest. If it is a document that you are creating for their use as one of many, you want to be creating a template. You could have AutoText entries in the template displayed by AutoText fields.