k2pdfopt doesn't convert to mobi.
But it *should* be able to reflow the PDF to fit better on a 6" screen. Ask in the k2pdfopt support thread for help finding the right parameters.
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And yes, direct conversion (and the OCR that entails) will be far more trouble than it is worth for a one-off conversion. Especially if math is involved!
There is a sticky in this forum to that effect.