How do I make solids transparent when printing to a PDF?

Hi all. I work in a landscape architecture office, on AutoCAD 2005. Often when making preliminary drawings, we color them in AutoCAD by using solid hatch pattern fills in various colors. I have the layers set up within my pen file to be somewhat transparent, and it prints like this to my HP plotter. However, when I generate a PDF, the color hatches are solid and you can't view the detail underneath the layer (for example- if a sidewalk goes underneath a tree canopy, the sidewalk is not visible underneath the tree in the PDF, but is visible in a print directly from ACAD.)

I've tried generating PDFs both using Acrobat 9 Pro as well as CutePDF Writer- results are the same either way.

Can anyone help on this? Thanks.