Crop PDF sets a new crop box on every page, pulling the four edges in by the amount you give. Drag the frame on the page preview, or type points or a percentage. A percentage is read against each page, so mixed sizes lose the same fraction. Every value starts at 0, so this shrinks a page and never adds margin.
Questions
What units are the margins in?
Either PDF points, where 72 points is one inch, or a percentage of the page. Points are what the format itself uses, so trimming 36 from the top removes exactly half an inch. Dragging the frame on the page writes percentages, because that is the unit that means the same thing to a Letter page and an A4 page.
Does cropping delete the content outside the box?
No. It sets each page's crop box, which is what readers and printers display. The trimmed material is still in the file, treat cropping as framing, not redaction, and use the redaction tool when the content must be removed.
What if I crop too much?
It stops and tells you which page and which numbers: cropping more than the page is wide or tall would leave nothing, so the tool refuses rather than writing a zero-size page.
Can pages of different sizes be cropped together?
In percent, yes: 5% is read against each page as the run reaches it, so a document mixing A4 and Letter, or upright and sideways pages, loses the same fraction from every page. In points it takes the same absolute trim off each, which is a bigger bite out of the smaller page. Pick the unit that matches what you mean.