Lineweight becoming too thin when file saved as a PDF

Howdy:

I'm drawing some coloring book pages for a client. They complained that some of the linework was too thin when they printed the page. So I went in and thickened the lines, saved the file as a PDF and re-sent it, and they said the lines looked exactly the same as before. I noticed the lines did indeed look thinner if I opened the PDF in Acrobat or if I placed it inside a Word file. Here's a screenshot showing the same file in the Illustrator window (left) next to the Microsoft Word window (right). You can see the lines in Word are thinner (it's especially noticeable in the little squirrel at the left, on the tree trunk).

Most of these lines are drawn with a brush and stroked, so I tried expanding all those lines and resaving to the PDF (at Press Quality)--no difference. I tried resaving as an EPS and importing that into Word--no difference.

I don't know how to make the lines the thickness they want, because what I see in Illustrator is not what they're getting in the end. I guess I could just make everything thicker than I think it should be, and hope the thinning in the conversion to EPS/PDF balances out, but that's a very annoying, slow workaround. Does anyone have any advice?

Is there some preference in Illustrator I need to change? Maybe the lines are being "artificially" thickened on my Illustrator artboard, the way some people are seeing too-thick lines in Acrobat because of the "smooth line art" preference? I don't know what preference that would be, though.