ImageMagick: Transparent Backgrounds for JPEG, not PNG - imagemagick

Question

I'm using ImageMagick to convert PDF files to PNGs. (Lots of text, so I'd rather use PNG over JPEG.) I'm doing this on OS X, 10.8.2.
I tried using Ghostscript, then ImageMagick, as was done here and I got a gray background. I shortened it to one line, since ImageMagick can work with PDFs and tried this:
convert -background transparent -transparent white \
Background-Page01.pdf TestClearX1.png
I've tried that with PNG files and JPEG files. (And with and without -transparent white as well.)
If it's a JPEG, I can get a white background (which is probably clear, but in my viewer, I can't tell), but with a PNG, I always get a dark background. I thought about, as a test, trying to generate a BMP, then converting that to PNG, but it won't generate BMP files.
How can I get a transparent background for a PNG file?
And if that's not possible, since JPEG files are not good for text, is there a better alternative?

Answer 1

This isn't an answer, but the format of the comment section doesn't allow sensible formatting, so I am offering it here instead.
You alluded to the fact that Preview in OS X doesn't show transparency properly, and that is correct. To get around that, I made the following script, which overlays the files you want to view onto a checkerboard pattern like Photoshop does. I save it as preview and then use chmod +x preview on it to make it executable. Here is the script:
#!/bin/bash
################################################################################
# preview
# Preview images using OSX "open" but overlay images on top of checkerboard
# first so you can see transparency/transparent pixels, since Preview renders
# them grey :-(
################################################################################
for f in "$#"
do
base=$(basename "$f")
composite -compose Dst_Over -tile pattern:checkerboard "$f" "/tmp/$base"
open -g "/tmp/$base"
done
That enables you to do:
/.preview file1.png file2.gif
and the -g option also means Preview doesn't steal focus too :-)
So, instead of looking like this:
it looks like this:

Answer 2

There's two ways to export a PDF in LibreOffice, and one way does not provide any warnings. The other way provides a warning that PDF/A cannot have transparent objects. The problem is you're using PDF/A files that don't support transparent objects—this results in your PNG always having a background colour.