I was going to say FastStone Capture but it is not free any more but you can still get the last free version.
But it will not open a .pdf file but it will open many image formants and you can then save in many formats and saving as a .pdf is one of them. So it does what you want but the other way around.
But try looking here.
http://lists.thedatalist.com/pages/PDF_Tools.php
You know it is not free but shareware butGraphic Workshop Professional does a whole lot of formats.
I have Graphic Workshop but not the Professional version and will look for it. I installed it years ago but then just copy the folder to a zip and can now run it from the zipped without having to install it again.
It too was shareware but has never stopped working. You get when you close the program down a voice that says please register GraphicWorkshop.
http://www.mindworkshop.com/alchemy/gwspro.html
Ok I just found mine and also from looking at the site I don't think it does .pdf but it sure does a lot of converting so let me take a look and it will not open .pdf. But hey after 10 years it still works.
http://www.mindworkshop.com/alchemy/gwspro.html
Ok IrfanView will open .pdf
http://www.irfanview.com/
Supported file formats in IrfanView
http://www.irfanview.com/main_formats.htm