Is it possible to set resolution on a PNG for tesseract 3.05.02?

[pbw]

I have generated PGM images from a pdf file, using pdttoppm -gray.  The clarity of these images is dramatically improved by setting a resolution of 300 or even 600. For tesseract, I convert these to PNGs.

Running against a PNG, I always see this:

Warning. Invalid resolution 0 dpi. Using 70 instead.

I have tried using IM convert to add a -density to the PNG, but it is not recognised by tesseract.  In any case, tesseract processes the image.  I have two questions.

1) Is it possible to set resolution on a PNG in a way that tesseract will recognise?

2) Would it make any difference to the tesseract result?

Thanks.