Should I use preflight to reliably check the resolution for a pdf file? I have posted my frustrations on http://forums.adobe.com/thread/978098
I believe Preflight is all you'll need. Going by the example you gave it would seem that there's an error in the conversion of the resolution for output. Are any particular errors displaying?
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For "digital" it is "PPI" (pixels per inch). For the umbiquitous imprints to paper via printers it is "DPI" (dots per inch) - from back in the day when attached printers were line printers and dot-matrix printers. As you dealing with "digital" (a PDF) "PPI" is appropriate.