A scan usually arrives with more paper than page — a white border, or a black strip where the lid did not close. On a phone screen that wasted edge is most of what you can see. Cropping trims it off every page in one step, so the document fills the screen it is read on.
Free on every plan, including the free account.
The file is read in the browser you are already using. Nothing is uploaded.
The same trim is applied to every page, which is what a scan of one stack of paper needs — all the pages have the same border, so they should lose the same amount.
The new PDF downloads to your device with the pages trimmed, so it looks right wherever it is opened rather than only in your reader.