Convert multipage PDF to a single image

Question

I have to convert a multipage PDF document to a .png image.

I try with ImageMagick, but I cannot achieve final result:

convert document.pdf document.png

or

convert -adjoin document.pdf document.png

This command create N images .png (where N=num of page of document):

document0.png
document1.png
.... 
document(N-1).png

I need, if possible with a single command, to obtain a single image.

Answer 1

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