What OS and what version of word are you using. Also what method are you using to create your PDFs?
Im using Word X on my mac here and word has a limitation of 0.56" for the bottom margin and 0.25" for the top/left/right margins. If i ignore the error it gives me about my margins, when I go to print or PDF it cuts off at those min margins anyways.
I believe its the same for Word 2000 for PC.
Word 2002(XP) and 2003 for PC they increased the min, I just made a file on my pc here using Word 2002 with 0 margins and put a solid box on the page, if I just use PDF writer it made my PDF with no margin errors and produced a PDF with my full bleed box.
If I make my PDF by printing a postscript and distill it I take on the minimum margins for the driver of the printer im using, so I get an error when printing telling me that I have objects outside of the margins of the printer, continue or cancel. If I distill that postscript I have margins of whatever the driver for my printer is.
Side note: I hate MS Office