PDFreactor does not create documents with a fixed resolution. Images are embedded at their original resolution while text is stored as references to the glyphs in the true type font, which makes it resolution independent. Therefore there are no "resolution" settings.
The resolution at wich PDFreactor lays out documents is 96 pixels per inch (as defined by CSS). This can be changed by using the API methods "setPixelsPerInch" and "setPixelsPerInchShrinkToFit". Keep in mind that these do not affect lengths specified in physical units like "inch" or "cm".