I've found similar problems with the Journal print driver (some PDF files are OK but others come out in a horrible resolution). The solution I have is to use the Microsoft office document imaging printer. (You'll only have this if you have office installed unfortunately).
You need to set the printing properties to print as a TIF at the maximum resolution (300 dpi). Then it's simply a matter of importing of importing the resulting TIF into Journal.
It's a shame that the otherwise excellent Journal has a poor printer driver -it would be great if MS could port the office team's one instead.