I figured it out - the PDF ActiveX outputs a 2bit black & white BMP that apparently windows/the printer driver see fit to interpolate or dither during printing. The same behaviour occurs even when printing from popular image-editing programs like Paint Shop Pro. If I pump the PDF BMP through an image handling ActiveX (LeadTools) and up the number of colors to 256 (8bpp), the image comes out perfectly!

The reason I did not choose to allow FoxPro to scale (isometric) the image is because a PDF varies in height and width depending on how many bytes of data it contains. If I allowed FoxPro to scale, the "bits" in the image would grow larger or smaller, possibly affecting scanner readability. Right now I just specify an oversized picture box in which I think the PDF will never outgrow and keep FoxPro's hands off the image.