Try updating the driver to a newer version for your printer.
And if you can get a Postscript Driver for your printer.
I had a similar issue last year and installing a postscript driver fixed the issue for me.
What printer are you using?
What PDF preset are you using? The problem is most often seen when you have transparency on the page and are saving to a flattened pdf format like PDFX-1A or PDF1.3 .
Hi, Thanks very much for both of your replies. It did turn out to be the pdf presets. The only one that worked was "[PDF/X-4:2008]..."
All fixed done.
I had my IT department install a Postscript Driver to the Printer and it fixed this (weird semi-bold) problem printing PDFs. Very Helpful!