Increasing trim size from 9" x 8.5" to 11" x 8.5" on PDF book

[Idony6C2B]

As the title says, I need help increasing a PDF book interior. It has entirely black background and I would like to simply increase the trim in black to make it fit a POD's trim size requirement (lulu) by adding a black border/space/margin.

Essentially the pages of the original PDF would be centered and black would fill the extra space bordering them to fit to 11" x 8.5" trim.

Is there any way of doing this or is this viable at all?

I am using Acrobat Pro DC, thanks for any advice.

[Brad @ Roaring Mouse · Accepted solution]

Obviously it would be better to do these changes in the originating document/application.

Failing that:

It's easy to change the page size in Acrobat: Use the Organize Pages > Set Page Boxes > Change Page Size.

As far as adding the Black background, you can use the Edit PDF > Add background, but you may not have the control you want.