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This is the result of the pdf being created using something other than pure black for text color. Some books use dark gray, and if text was in color it'll be mapped to some shade of gray on grayscale devices.
There is no control built in to the kindle ecosystem to adjust this. If you are creating the pdf, you can adjust the color of the source text. If you are dealing with an already created pdf, then you will need a pdf editor. The gold standard here is Adobe Acrobat (retail $300). For something a little cheaper the folks who do the free Nitro pdf reader also sell an affordable pdf editor.
You might be able to use Calibre, but it is primarily a format converter to/from pdf and other ebook formats. Experience with pdf's on Calibre has not been very good for anything other than the simplest of texts.
Hope this helps a bit. Amazon still has a ways to go on pdf handling.
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