Some PDFs consist of images, one per page. Essentially, the PDF is just a wrapper around a collection of images. When converted for the Kindle, these most likely end up as an AZW wrapper around the same collection of images. So there is no actual text to rescale.
If this is the case here, you can either use the Kindle's native PDF to zoom (at bit) in landscape mode or try one of the image-based PDF converters. See the sticky threads in the PDF forum and also
PDFRead 1.8 released!