You might want to try Mobipocket Creator to convert the PDF's, sometimes the results will be better. However PDF is probably the worst format to do any conversions from.
Are the PDF's not usable as is on the K2?
My best results for converting a PDF for the K2 has been to use Acrobat Pro and export as HTML then open the HTML in Sigil and create a nice ePub, then convert the ePub to Mobi in Calibre.
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