how to detach the content

You kinda can, and kinda can't.

Let's start with the bad part first. Acrobat's crop tool really is a crop tool. Your PDF normally remains the same size, and the same dimensions, as originally created. The crop tool essentially creates a compound mask over the file, providing a revised window for how your PDF file appears.

The other two answers here 1) reaffirm that you essentially can create a smaller-dimension PDF if you insist by using the Print to PDF function to re-compose your larger PDF to smaller dimensions (try67). But it's a destructive change and can't be undone. So for documentation/safety's sake, you may want to create a copy of the original PDF to smaller dimensions rather than replace it. You also 2) can place the cropped PDF in other applications (InDesign, Illustrator, etc.) without concern(JR Boulay). It will place the PDF document at full size, but display only your cropped dimensions in the application. As long as you can avoid selecting the placed element with the White/Direct-Selection Arrow tool, you should be safe to scale it to your heart's content.

Hope this helps,

Randy

In Acrobat Pro choose: Edit

Using the Select tool clic and drag to select all the objects to be resized, then clic on a corner of the whole frame and drag to resize.

There's no such option in Acrobat. PDF files were not really meant to be edited in this way.
What you can do, though, is crop out that part of the page and then print it out to the Adobe PDF printer to create a new file with new page dimensions.