Change PDF’s physical dimension or updates its cropbox.
The cropping parameters are specified in PDF / postscript points. You can use Preview’s inspector tool to extract the right selection parameters.
Notice the crop size may be different from the page size. Updating the cropbox will update PDF’s ‘view’.
What is cropbox?
The CropBox defines the region that the PDF viewer application is expected to display or print. So if a PDF contains a CropBox definition, Acrobat uses it for screen display and printing. For prepress use, the CropBox is pretty irrelevant.