The graphic is likely a pixel based/image format.
Images are downsampled upon pdf creation to the anticipated need.
If you chose Standard Quality for the pdf, the image graphic is at 72 pixels per inch, much less than suitable for anything other than images on screen.
If the graphic incorporated type or sharp lines and edges, a minimum suggested pixel per inch setting would be 600.
Choosing High Quality as the pdf preset may suffice.
We would need to know the type of image used in the Word document. A screenshot would help.