You can adjust the visible page area using the Crop Pages tool and the Set Page Boxes dialog box. Cropping pages can help you create consistency within a PDF composed of pages of different sizes.
Cropping does not reduce file size because information is merely hidden, not discarded.
The Set Page Boxes dialog box contains options for cropping pages. The Margin Control options are as follows:
Show All Boxes (Acrobat Pro DC)
Shows the black, red, green, and blue rectangles indicating the CropBox, ArtBox, TrimBox, and BleedBox on the preview. When two (or more) margins coincide, only a colored line appears.
BleedBox (Acrobat Pro DC)
Defines the clipping path when the page is printed professionally to allow for paper trimming and folding. Printing marks may fall outside the bleed area.
Locks the proportions of the crop so that all margins are at the same distance.
Remove White Margins
Crops the page to the artwork boundary. This option is useful for trimming the edges of presentation slides saved as PDFs.
Cropping a PDF does not reduce file size because information is merely hidden, not discarded. By resetting the page size, you can restore the page and its content to its original condition.