command-line wildcards conversion imagemagick – Converting Multiple JPEG Files to PDF Format

You can check-out pdfdraw from mupdf (package mupdf-tools under Debian/Debian-derivatives).

From its description:

pdfdraw will render a PDF document to image files. The supported image formats are: pgm, ppm, pam and png. Select the pages to be rendered by specifying a comma separated list of ranges and individual page numbers for example: 1,5,10-15). In no pages are specified all the pages will be rendered.

Perhaps it is faster for your use case.

For example mupdf (a PDF viewer) is really fast (and consumes very little memory) for a lot of documents I tested it with.

Use the convert command from ImageMagick:

convert x.png -resize 25% x.jpg

will create a 25%-scaled JPEG version of x.png.

To do all the PNG files at once, use a for loop:

for filename in *.png ; do convert "$filename" -resize 25% "${filename%.png}.jpg" ; done

We look at every file matching *.png, and for each one run the convert command above. "${filename%.png}.jpg" generates the .jpg filename for each .png: it removes the extension with % and puts .jpg back on the end.