NOTE: Please read the comments, as well as my “Later Notes” at the end of this post. While this post and the solutions offered by commenters all provide improved methods for cropping PDFs, there are still some catches.
If you’ve ever used the Crop tool in Acrobat, you’ve probably discovered that it doesn’t really crop: it just masks out content. Other applications may ignore the mask and reveal the stuff you were trying to delete. And since content is not deleted, there’s no reduction in file size.
Here, I’ve added a big honking half-inch bleed in Illustrator to make it obvious. The red border is the bleed (natch).
Let’s say I want to delete the bleed, just leaving the trim area of the PDF. I could use the Crop tool, but that’s not exact. Instead, knowing that I have 0.5″ bleed, I launch the Set Page Boxes tool under the Print Production tools. I choose CropBox from the pulldown at the top of the dialog, and enter 0.5″ in all the Margin Control fields. When I click OK, it appears that I have cropped the PDF to the trim. Whee!
But heartbreak awaits me when I return to the Set Page Boxes dialog to check it…Doh! The rind isn’t gone! Like a persistent zombie in a bad horror flick, it’s still there.
But don’t despair: there IS a way to kill the zombie. Usually, that requires a head shot, but in Acrobat, your weapon of choice is a Preflight Fixup. If you’re lucky, and you’re trying to just trim the bleed off a PDF made from Illustrator or InDesign, both of which explicitly define the trim and bleed boxes in PDFs, it’s a fairly easy fix. Launch Preflight in the Print Production tools, then select the Select Single Fixups icon (circled).
In the Create Fixup dialog, name the new Fixup, and choose the Pages option in the left column. On the right, choose Set page geometry boxes. In the bottom half of the panel set Source to MediaBox (meaning “get ready to change the imaginary piece of paper”), and for Destination, choose Relative to TrimBox (meaning “use the dimensions of the TrimBox”). Click OK to return to the main Preflight panel. This just creates the Fixup; you still have to set it in motion. Make sure that the new Fixup is selected, then click the Fix button.
The bleed appears to be gone, but the paranoid in you (printing will do that to you) will no doubt drive you to double-check. Relaunch the Set Page Boxes function and squeal with joy: it really is gone—at least as far as Acrobat and most other applications are concerned.
I’m finding that all of these approaches seem to delete extraneous content as far as Acrobat is concerned. This is true of the method I’ve shown, the Preflight Fixup that Jean-Claude mentioned, and the Resize Pages script. However, if you “de-crop” the PDF in Acrobat by changing the page size to the original dimensions (enlarging the canvas, either via the Crop tool or the Set Page Boxes function under Print Production) the content is STILL THERE! Aargh! It can’t be killed. You can also prove this to yourself by opening up the “cropped” PDF in Illustrator—nothing has truly been deleted.
Apparently PDFs are made of some sort of indestructible material; parts can be hidden, but not truly destroyed. However, despite this seeming disappointment, I think you’ll find that any of these methods will result in PDFs that behave as if they’re cropped, if they’re placed into other applications, such as InDesign. Try to not think about the lurking hidden material.