It's not clear what you mean by "quality loss". That could mean a lot of different things. Could you post some samples to illustrate? Perhaps cut the same section out of the poor quality and good quality versions (as a PNG to avoid further quality loss).
Perhaps you need to use
-density to do the conversion at a higher dpi:
convert -density 300 file.pdf page_%04d.jpg
(You can prepend
-units PixelsPerInch or
-units PixelsPerCentimeter if necessary. My copy defaults to ppi.)
Update: As you pointed out,
gscan2pdf (the way you're using it) is just a wrapper for
pdfimages (from poppler).
pdfimages does not do the same thing that
convert does when given a PDF as input.
convert takes the PDF, renders it at some resolution, and uses the resulting bitmap as the source image.
pdfimages looks through the PDF for embedded bitmap images and exports each one to a file. It simply ignores any text or vector drawing commands in the PDF.
As a result, if what you have is a PDF that's just a wrapper around a series of bitmaps,
pdfimages will do a much better job of extracting them, because it gets you the raw data at its original size. You probably also want to use the
-j option to
pdfimages, because a PDF can contain raw JPEG data. By default,
pdfimages converts everything to PNM format, and converting JPEG > PPM > JPEG is a lossy process.
pdfimages -j file.pdf page
You may or may not need to follow that with a
.jpg step (depending on what bitmap format the PDF was using).
I tried this command on a PDF that I had made myself from a sequence of JPEG images. The extracted JPEGs were byte-for-byte identical to the source images. You can't get higher quality than that.