PdfEdit -- the strongest open source PDF editor.

There is no budget for me to study into Acrobat's pdf editor in detail, so, I do not know whether PdfEdit is weaker or stronger than it.

But based on my experience, I just recommend PdfEdit as your pdf editor, especially if you do not bother do a little script programming.

Here, I give an example of how could you change all pages' size with PdfEdit.

Step1: goto 1st page, and decide the desire page size: In this example, we change the left margin to 50, the effect on 1st page is:

Step2: goto 1st page again, if you want to change page 1-6's size to (50,0,595.2,841.92), then input the scripts to script window and press enter to execute it, that's all:

for(i=0;i<6;i++)

{
page().setMediabox(50,0,595.2,841.92);
PageSpace.nextPage();
}
page().setMediabox(50,0,595.2,841.92);

For more samples of PdfEdit script, you can refer to .ps files in PdfEdit home dir, e.g., ubuntu install it to /usr/share/pdfedit/.
Detailed script API can be found at: http://pdfedit.petricek.net/appendix.html