mogrify ImageMagick and PDFs

We've been using ImageMagick for some time to create thumbnail sheets from .TIF images. This line has been working hard for months without problems:

usr-bin-mogrify -size 180x180 -resize 180x180 -quality 85 -colors 256 -colorspace RGB +profile"*-"var-www-html-pa-thumbs-shipthumbs-*.tif

(That turns the images into uniform sized JPGs after which the script runs:

usr-bin-montage -tile 4x4 -colorspace RGB -label "%f" -size 180x180 -resize 180x180 -geometry 180x180+0+0 -gravity Southvar-www-html-pa-thumbs-shipthumbs-*.*var-www-html-pa-thumbs-shipthumbs-montage.jpg

which makes the thumbnail sheet. But for this PDF testing I'm just dealing with that first, mogrify line.)

But now we need to add .PDF's to the source files for the thumbnails. I gave it a try, not at all thinking it would work, by replacing *.tif with *.pdf. But it sort of did work.

Of the two test PDFs, one actually was converted to a JPG, but instead of being 180x180 it was more like 80x80. The 2nd test image was simple a large black square.

I tried this:

usr-bin-mogrifyvar-www-html-pa-thumbs-shipthumbs-*.tif

just to see how different it would be and go from there...but in fact, it created the same results! A tiny but OK looking JPG from PDF 1, and a large black square for PDF 2. There doesn't appear to be any major differences between the two PDF's. Both have layers (orgres have layers! Sorry.) Both are CMYK color, both are the same dimensions and resolution.

Is there some way to get mogrify to work with PDFs better, or will I have to resort to PDFLib or something? (Yikes!)

Thanks for any feedback! Liam