Determining dimensions of a PDF without having Aunt Illy open it.

Command D on a Mac , FYI... 🙂

It's also useful to know the final intended size of the PDF, AKA the trim size. In Acrobat (not Reader) you can use the measure tool to check the distance between crop marks, or a faster way is under tools> Print Production> set page box (select trim box). This will give you the exact trim size. You can also go to Acrobat preferences> page display> show art, trim and bleed boxes. This is a good way to confirm bleed has been included in the pdf.

Seems like you don't want to open Illustrator but not that you wouldn't open Acrobat or Acrobat Pro?

Hover your mouse over the bottom left corner of an open file in Acrobat and you have the dimensions in a box that appears.

You mean without opening the pdf in any program including  Acrobat? Viewers that can read the metadata like Bridge in its Metadata tab will have the size of the Artboard in pixels. You have to divide by 72 to find inches.

In Acrobat, aside from displaying the rules (Ctrl + R) you can always File > Print (Ctrl + P) to see the print dialog or move the mouse at the bottom of page. The size there is displayed in inches and there is also a few other places in Acrobat where the size is displayed as Document Properties from the file menu and the the Crop page dialog.

edit: Also you can open the pdf with a web browser and hit print (Ctrl + P) or move the mouse at the bottom of page to display size