Images in PDF do not have an explicit alpha channel. Instead there may be a soft or image mask associated with the base image (that may or may not be of the same dimensions as the base image; for more info please see Section 8.9.5 in PDF Reference: http://xodo.com/view/#/c0c11968-ee14-478e-9b09-6dc5635c0915). You can learn more about SoftMasks from PDFNet KB: https://groups.google.com/forum/#!searchin/pdfnet-sdk/SMask
If you want to rasterize a PDF page with transparent background (rather than solid white paper background, please see https://groups.google.com/d/msg/pdfnet-sdk/GFqayLaJdSU/J_0gqGvcAmAJ