I have an answer. (a) It is a kludge. (b) It will be tedious, if you have a lot of equations, unless you can automate it. (c) It might not work if any of the equations is more than about 4 inches wide.

- Go through the document, select every equation, and change its font size to double (or larger, if you want, or smaller, if you have to). If you have inline equations that break across lines because of this, add line breaks to avoid it. If you have equations that are more than 3¾ inches wide (7½ ÷ 2), set the left and right indents for that paragraph negative. If you have equations that are more than 4¼ inches wide (8½ ÷ 2), put the document into landscape mode, or set a custom paper size. It’s OK to scale different equations by different scale factors, but just keep track of the factor for each equation.
- Save the
`.docx`

as a `.doc`

.
- Go through the document, select every equation image, and change its size to half (or, in general, the inverse of the factor by which you enlarged it). You may find inline equations appear sunken:If so, select them, go to the “Font” dialog box, “Character Spacing” tab, and adjust the “Position” until it’s right. (Start by multiplying it by the same reduction factor that you used for the image; e.g., half.) Undo any damage that you did to the document; e.g., extra line breaks, negative margins, undesired page orientation and/or size.

In this example, I created a standard equation in my default Normal font, which is 11 pt.
Then I made a copy of it (for illustration) and changed it to 22 pt. Then I saved as a `.doc`

, made a copy of the second image (the larger one), and scaled it back down to 50%.

It seems to me that the third image in the `.doc`

is less blurry than the first one.
Presumably it would get even better if you could use more extreme magnification/reduction ratios.