When I save a Word document as PDF, the PDF file seems to get a standard "initial view" setting. Is it possible to change that so that created PDF files open with Magnification=Fit Page?
No, it is not possible to control the Initial View settings for the PDF being generated when exporting a PDF from Word using the built-in Save As PDF feature.
These settings can't be modified in Adobe Reader unfortunately so you will need to find a PDF editor that lets you change the settings. There's a few free options, one of them is Debenu PDF Tools, a free utility for manipulating PDF files (that is provided by the company I work for).