Rotate PDF, Crop PDF, and Resize PDF all sit in the same "fix how this page looks" category, and it is genuinely easy to reach for the wrong one when a page just looks wrong and the specific reason is not immediately obvious. Each tool changes a distinct property of a page, and once those three properties are clear, picking correctly takes about two seconds.
Rotate PDF changes a page's orientation — turning it 90, 180, or 270 degrees — without touching its content, dimensions, or what is visible on it in any other way. This is the tool for the extremely common scanning mistake where a document fed into a scanner sideways or upside-down comes out with every page rotated the same wrong way. pdf-lib handles this by updating the page's rotation property directly, which most PDF viewers respect natively; the underlying page content stays exactly as it was, just displayed at a different angle. If a page's content is right-side-up but the whole page is turned the wrong way, this is always the fix.
Crop PDF changes what portion of a page is visible by adjusting its visible boundary, effectively trimming away margins or unwanted edges without touching the actual content's size or position on the underlying page. This is the tool for removing excessive white margins around a scanned document, trimming off a printer's crop marks or registration guides before sharing a file, or focusing a page down to just the content region when the surrounding whitespace is distracting. Cropping does not shrink or enlarge anything — it changes which part of the page counts as "the page" for viewing and printing purposes.
Resize PDF changes a page's actual dimensions — converting between standard sizes like A4 and US Letter, or scaling a page up or down to a specific target size — which does affect how large the content appears, since the content is scaled along with the page it sits on. This is the tool for the specific, common problem of a document created at one regional paper size needing to print correctly at a different one: an A4-formatted document intended for a US Letter printer, or vice versa, where naive printing either cuts content off or leaves it awkwardly small with excess margin.
A concrete way to tell them apart when a page looks wrong and the specific issue is not obvious: if the content itself is oriented correctly but the whole page needs to be turned, that is Rotate. If there is unwanted empty space or unwanted content at the edges that should simply not be part of the visible page anymore, that is Crop. If the page as a whole needs to become a different physical size — usually for print compatibility across regions or systems — that is Resize.
All three tools share the same underlying honesty: they are layout operations, not content operations. None of them can fix text that is genuinely too small to read, recover content that got cut off during an original scan, or correct a document that was authored incorrectly from the start. What they can do, reliably and entirely locally, is fix the specific, common class of page-geometry problems that come from scanning, cross-regional printing, and inconsistent source formatting — which, in practice, covers most of what people actually run into.