I have yet to have a PDF version of a W9 form allow me to fill it in completely without telling me some portion of the thing will not allow a text field and/or signature. It's a real fucking pain in the ass, is what it is.
I've found that Word tends to introduce problematic (and invisible) formatting codes at times -- I tend to just print out the original PDF, scan/OCR it into a fresh PDF, then edit it by hand in Acrobat. When I'm feeling grumpy, I do the scan at really low resolution. . . .