Upload the PDF with annotations and the PDF that should receive them. The annotations must be live/editable, a flattened export won't work. Annotations are transferred only to the same page number and coordinates, so this works best when the receiving PDF keeps the same layout. If a receiving page uses a different page size, it is resized to match the annotated PDF before transfer.
For larger PDFs, the download may start after 10 to 20 seconds.
If the PDFs have different page counts, annotations are transferred only on overlapping pages.
Resizing a receiving page can discard any existing live annotations on that page.