How can I stop Adobe Acrobat Pro XI from making my footers different sizes?

When combining various Pdfs and Word documents into one Pdf file I like to put a page number footer on each page. However, sometimes when I do this, the page numbers come out in all different sizes. I just want to overlay the footer over each page, regardless of what the page shows, and don't want Adobe changing the font sizes by itself.

Any way to stop this from happening?

Thanks