Can I undo a deletion?

Yes, click a page again to deselect it before applying. After download just upload the original again.

Is there a page limit?

No fixed limit, though very large PDFs may take longer to render thumbnails.

Internal bookmarks pointing to remaining pages are preserved. Links to deleted pages are removed.

No. Pages are removed in memory and the file is discarded after download.

Click pages to mark them for deletion, then download the trimmed PDF