Thanks for the suggestion. I can't get this approach of expanding the bottom margin to compensate for the drift 100% of the time. It works on some pages but not others. The problem is that the amount of drift varies based on the formatting on the page. Some pages have section headers with larger fonts or other paragraph formatting so the amount the bottom margin needs to be expanded varies from page to page, and document to document.
The problem is caused by the fact that line height in the PDF is very slightly smaller than the line height in the original word document. Each line drifts up on the page very slightly compared to the Word document causing an accumulated drift of several lines by the end of the page.
Can't you just fix the problem by maintaining line height exactly?