Word on the street is that Visio 12 will have pdf-support built in.
It's true -- I've played with the early implementation of "Save as PDF" in
Beta 1 of Visio 12 (and as of last week, am allowed to talk about it!). Of
course, that doesn't solve your immediate problem, but other posters have
mentioned that various paid/shareware/freeware pdf writers exists. Go to
tucows.com or shareware.com or any similar site and search for "pdf writer".
You'll probalby find more than you want!
One caution: if you have embedded hyperlinks in the drawing and want them to
work in your PDF, you may need to try several of the PDF writers before you
find one that works.