Check your convert to PDF. (I don't have MS Word so can't tell you want options it has.) Some have a specific option for "Reduce image resolution" (that's what LibreOffice says), others have a simple tick box that might say something like "Print Optimised" or similar, and when this is not ticked the conversion will reduce graphics (or it may be a reverse logic "Screen Optimised" tickbox).
(Maybe someone that actually has MS Word can be more precise than this vague offering.
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