Convert multipage PDF to a single image

Finally I find THE solution:

convert in.pdf -append out%d.png

Thanks to this post.

edit

As a plus, the opposite operation is:

convert *.png output.pdf

or if you have foo1.png, foo2.png..fooN.png

convert foo?.png output.pdf

Notice that does not work with foo01.png, foo02.png..foo0N.png