I use Adobe Acrobat, which is hardly free, or sometimes GhostScript and The GIMP on Linux.
If you are a handy coder and want to play, you could try this rather nice .NET solution: click here
There are also ways of using PHP, but there isn't enough space in this forum to go through the necessary steps, code and the required PDF server support. You could say that it's a bit involved and, frankly, the return on the time investment is hardly worth it unless you have hundreds or even thousands of PDF files.
Off the top of my head I'm taking a guess that Adobe Photoshop could batch process this. Photoshop can open PDF files. You get to select which page of a given file you wantt o view and you can edit it in Photoshop. Note that this creates enormous files by comparison to Acrobat, but it can be done and Photoshop has a very nice batch editor so you could feasibly create your batch process and run it on a directory of PDF files.
A friend of mine (yes I do have one!) suggested the Pdf995 Suite to me a while ago but I've not had the time as yet to dip my toe: click here
Aside from that, nothing springs readily to mind that would be in any way simple.