With the Apryse library you can crop pages manually or automatically, as well as reset document pages to their original size. The UserCropDialogFragment allows a user to adjust the page's crop box which can then be applied to a subset of pages or to all the pages in a document.

To show a user crop pages dialog in your activity, create a new instance of UserCropDialogFragment by calling newInstance() and setting the PDFViewCtrl:

public void ShowUserCropDialog()
{
    var userCropDialogFragment = pdftron.PDF.Controls.UserCropDialogFragment.NewInstance()
        .SetPdfViewCtrl(mPdfViewCtrl);
    userCropDialogFragment.SetStyle((int)DialogFragmentStyle.NoTitle, Resource.Style.PDFTronAppTheme);
    userCropDialogFragment.Show(this.SupportFragmentManager, "user_crop_pages_dialog");
}

In order to view user cropped pages in PDFViewCtrl make sure you set Page.e_user_crop as the default page box:

mPdfViewCtrl.SetPageBox(pdftron.PDF.Page.Box.e_user_crop);