Let's take a look at a PDF with these incorrect blacks. In Acrobat, go to the Print Production pane > Output Preview.
I moused over the large black portion and the ink values are shown below. Notice how the black is actually composed of all four colors of ink.
1. Go to Print Production pane > Convert Colors.
That will bring up this dialog box. For our purposes, there are only two things to be concerned with in this dialog box. The first is the Conversion Profile. You can choose from a variety of CMYK conversion profiles, but SWOP is a good bet. And fortunately, it is the default here in the United States. Be sure to use a profile appropriate for your country.
The second thing to be concerned with (and this is the most important part), is the "Preserve Black" checkbox. According to the Adobe website, "Preserve Black preserves any black objects drawn in CMYK, RGB, or grayscale during conversion. This option prevents text in RGB black from being converted to rich black when converted to CMYK."
Click ok, and you are done. Now when you open the Output Preview dialog box and mouse over the back area, notice how the blacks are corrected.
And when you turn off the black plate, all the blacks disappear, as they should. Now the document will print properly.
I hope this tutorial will help you more quickly convert the RGB blacks in your Microsoft Office PDFs.