The currently accepted answer does the job but results in an output which is larger in size and suffers from quality loss.
The method in the answer given here results in an output which is comparable in size to the input and doesn't suffer from quality loss.
TLDR - Use
pdfimages -j input.pdf output
Quoting the linked answer:
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
convert -density 300 file.pdf page_%04d.jpg
(You can prepend
-units PixelsPerInch or
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).
does not do the same thing that
convert does when given a PDF as
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,
everything to PNM format, and converting JPEG > PPM > JPEG is a lossy
pdfimages -j file.pdf page
You may or may not need to follow that with a
(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.