[poppler] pdftoppm: resulting image dpi and/or size is unexpected

[Carlos Fernandez]

trying to split a pdf into page images using pdftoppm.  I would like
the resulting images to maintain the same size and scale as the source
PDF.  unfortunately the images being generated are somewhat
unexpected.

using "-r 150" (or just relying on the default value of 150) to create
an image on a 8.5x11 inch pdf generates an image that is 1275 × 1651
pixels at 150dpi.

using "-r 240" to create an image on the same 8.5x11 inch pdf
generates an image that is 2040 x 2640 pixels at 239.97 dpi.

these results were consistent for png or tiff output.  I got similar
results for other dpi settings across a few different PDFs.

What can i do to get more consistent results?

Carlos