You can adjust the visible page area using the Crop 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.
To crop a page with Crop tool in Acrobat DC refer: Crop PDF pages
The Set Page Boxes dialog box contains options for cropping pages. The Margin Control options are as follows:
Show All Boxes (Acrobat Pro)
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)
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.
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.