Scans often arrive with a broad white border, and academic papers give up a surprising amount of every page to margins. Both are easy to trim back.
Every PDF page carries a box that tells a reader which part of it to display and print. Cropping moves that box inward. The content outside it is still in the file, it is simply no longer shown.
The good consequence is that this is reversible and adds nothing to the file size, so an aggressive crop costs you nothing and can be undone later. The important one is that it is not a way to remove something sensitive from the edge of a page, because the content remains in the file for anyone who looks properly.
If a scan is skewed rather than merely wide, crop after straightening, not before. Cropping a tilted page to a tight margin clips the corner of the text on the side it leans towards.
A trim of about 10 percent suits most scans and papers. Step through a few pages in the preview, in particular any with page numbers, footnotes or a wide table, because those sit closest to the edge and are the first to be clipped.
You can crop all pages the same, or name specific pages in the box. Leave it empty for the whole document. Take care typing a page range, because if the box holds nothing the tool can read as a page number, it crops every page.
No. It changes what is shown, not what is stored, so the size stays about the same.
Yes. Set the trim, name those pages, and save. Run it again with other settings for a different set.
No, and you should not try. The content is still in the file. To remove it properly, export the pages as images.
Start with about 10 percent and check a few pages. Raise it only while the text and the page numbers stay clear of the edge.